These are new slots; they can be used anew.
No text anywhere in the Warlock spellcasting section nor the general Spell Rules indicates that a spell slot ever changes levels. Given that no other class has level-changing slots, and this unique rule isn't mentioned in any Warlock write-up, the most direct reading of the rules is that they lose their 3rd level slots and gain 4th level slots instead at that level.
If slots "gained levels", then a Wizard going from 2nd level to 3rd with all expended spell slots should have already expended his new 2nd level slots - two of his first level slots leveled up to second, and he gained three new first level slots to bring him up to the amount in the table. Obviously, that's not what happens - the slots at the new level are new slots.
Like many other recent questions, though, this situation could be simplified by having the party level up during a long rest, so the slots are all refreshed anyway. In my personal opinion, I think leveling up during a long rest was an assumption in the rules that never got written in, so now there are all these questions. Can't prove that, though.