The Warlock Pact Magic feature says:

The Spells Known column of the Warlock table shows when you learn more
warlock spells of your choice of 1st level and higher. A spell you
choose must be of a level no higher than what’s shown in the table’s
Slot Level column for your level. When you reach 6th level, for
example, you learn a new warlock spell, which can be 1st, 2nd, or 3rd
level.

Additionally, when you gain a level in this class, you can choose one
of the warlock spells you know and replace it with another spell from
the warlock spell list, which also must be of a level for which you
have spell slots.

Consider a single-classed warlock leveling up from level 4 to level 5, where their Pact Magic spell slots change from 2nd level to 3rd level. They can replace a warlock spell they know with a spell from the warlock spell list which "must be of a level for which they have spell slots," and they don't have 2nd level spell slots anymore. Can the warlock therefore only replace a warlock spell they know with a 3rd level warlock spell? Or can they learn a new 2nd level warlock spell instead?

(The word "also" suggests the same conditions apply as in the previous paragraph, but the first paragraph uses different wording -- "a level no higher than what's shown in the table's Slot Level column," and the second paragraph refers to "a level for which you have spell slots.")

I am aware that this answer claims the interpretation that you can learn lower-level spells, but it doesn't give any justification for that interpretation or discuss the specific wording.

The Warlock's spell slot level indicates the maximum spell level they may select.

I agree the wording is a little confusing but consider the general rule for spell slots:

When a character casts a spell, he or she expends a slot of that spell's level or higher, effectively "filling" a slot with the spell. You can think of a spell slot as a groove of a certain size--small for a 1st-level slot, larger for a spell of higher level. A 1st-level spell fits into a slot of any size [...]

In other words, the phrase "for which you have spell slots" more fully means "for which you have spell slots that the spell can fill."

The rules for Warlock spells known suggests newly gained spells for a Warlock may choose spells that are no higher than what is shown on the table for their level:

The Spells Known column of the Warlock table shows when you learn more warlock spells of your choice of 1st level and higher. A spell you choose must be of a level no higher than what's shown in the table's Slot Level column for your level.

Additionally, when you gain a level in this class, you can choose one of the warlock spells you know and replace it with another spell from the warlock spell list, which also must be of a level for which you have spell slots.

The wording here explicitly states that newly learned spells must be "no higher" than the slot level indicated for your current level on the warlock table.

The subsequent paragraph goes on to say that if you swap a spell out for a new one, that new spell must "also" be of a level for which you have spell slots. The word "also" indicates that it follows the same rules for the new spells (ie, equal to or less than the slot level on the table).

Another way of thinking about this: Classes that must prepare spells from a larger list (Druid, Cleric, Wizard) are told "The spells must be of a level for which you have spell slots." It's the same "for" wording, but no one would argue that these classes must prepare spells of equal (base) level available to them. They are free to choose only lower level spell and upcast them using the available higher level slots.

Altogether, the rules indicate that a Warlock is free to choose any spell that is of a level that is equal to or less than the slot level indicated on the Warlock table, regardless of whether that spell is swapped in or learned in addition to the previously known spells.

RAW, they only have slots at the highest level and therefore can only choose new spells from that highest level

This is a weird scenario. Looking purely at the rules as written, Warlocks don't have lower level spell slots. They only have the spell slots at the level the Table determines.

Unfortunately as you've quoted, this does mean that when swapping spells that they are limited to only adding a spell at that spell level and spells at a lower level aren't available anymore.

Whether this makes sense thematically or not is questionnable - but that is the strict interpretation of how pact magic works.

This is somewhat supported in that they have to upcast any lower level spell when they cast it.

The case of the term "also"

In the section regarding this particular clause it states:

Additionally, when you gain a level in this class, you can choose one of the warlock spells you know and replace it with another spell from the warlock spell list, which also must be of a level for which you have spell slots.

It is unclear what the "also" means here, but if the first part of the sentence is stating that you can choose a warlock spell you know and replace it, then the also is likely an additional requirement - and that requirement is that it "must be a level for which you haves pell slots". And that level, for a warlock, is singular.

This may be a case of general vs specific. Where the first part of the section is the general rule for picking brand new spells, while the second part under question is the specific rules for exchanging spells.

But that's crazy!

Yeah, it kinda is. I don't think it would be gamebreaking for a DM or table to consider that WoTC likely meant it to be inclusive of lower level spells when levelling up. But you'll need to discuss with your DM and get their approval to move beyond the strict RAW.

Like most sentences in English, there is more than one way to read it.

The first is that also refers to the spell selection restrictions in the previous paragraph.

The Spells Known column of the Warlock table shows when you learn more warlock spells of your choice of 1st level and higher. A spell you choose must be of a level no higher than what’s shown in the table’s Slot Level column for your level. When you reach 6th level, for example, you learn a new warlock spell, which can be 1st, 2nd, or 3rd level.

Additionally, when you gain a level in this class, you can choose one of the warlock spells you know and replace it with another spell from the warlock spell list, which also must be of a level for which you have spell slots.

So also is followed by enough information for you to find that rule, in the previous paragraph, which also applies here.

This rollercoaster is awesome, and you should feel awesome. For safety reasons, you must be between 3'6" and 4'2" to ride this. When you finish the ride, someone will hand you a rubber chicken.

There is a second track attached to the ride. No electronics are permitted on the second track and you must also be of sufficient height to use the second track.

This is an example of this technique -- the also summarizes and refers back to the height restrictions in the first paragraph here, rather than repeating them.

The second valid reading is that also is internal to the paragraph:

[...] you can choose one of the warlock spells you know and replace it with another spell from the warlock spell list, which also must be of a level for which you have spell slots.

Here, also describes an additional restriction to what kind of spell you can choose to replace a warlock spell. Ie, it must be "another spell from the warlock spell list" and "level for which you have spell slots".

In this reading, you can only learn spells that exactly match the level of the spell slots you have.

Amusingly, here the wording doesn't say "warlock spell slots", so by that reasoning you if you are a Paladin 2/Warlock 7 you could pick up level 1 Warlock spells, because you have level 1 spell slots from Paladin 2.

To be clear, both of these are perfectly valid readings of those paragraphs. To determine which one is correct you'd have to use context.

My personal rule for interpreting ambiguous rules is to (a) determine if either interpretation is crazy unbalancing, (b) determine which interpretation makes bookkeeping annoying, and (c) determine which gives players the most freedom.

If neither is unbalancing, and neither makes bookkeeping suck, and one gives the player more freedom, I go with that interpretation. Which is the first interpretation.