As a high-level Warlock whose spell slots are 5th level I can have Warlock spells up to 5th level. But there are one or two 1st level spells that, while they don't scale with spell slot level, are so handy I haven't let them go.
But now I'm going to multiclass into a spellcasting class and get some 1st level spell slots along with the spellcasting feature. This new class has my favorite 1st level warlock spell on its spell list as well, so I'd obviously like to have my new class use that spell for one of my spells known and replace the Warlock spell known with a 5th level Warlock spell. I am struggling with how, especially by RAW, I can do this efficiently.
I can trade that spell away on my last Warlock level up, go a whole level without the spell, and then take it again when I multiclass. Or I could take it in my new class and have two versions of the same spell for a full level, and then trade out the Warlock version when I level up Warlock again. But ideally I'd make the swap right when I multiclass or the next time I level up as a Warlock without having either an overlap or a gap in spell coverage. And I'd like to do this in an ironclad, no trading "bending the rules for DM favors", way.
Is this possible and I'm missing it, or is there no way to do this?