Several spellcasting classes have a clause saying "when you learn more [class] spells of your choice, each of these spells must be of a level for which you have spell slots". They also have a clause saying "when you gain a level in this class, you can choose one of the [class] spells you know and replace it with another [class] spell of a level for which you have spell slots".
When multiclassing, you may gain spell slots by adding together your levels, but you are restricted by the following rules about multiclassed spellcasting:
You determine what spells you know and can prepare for each class individually, as if you were a single-classed member of that class.
If you have more than one spellcasting class, this table might give you spell slots of a level that is higher than the spells you know or can prepare. You can use those slots, but only to cast your lower-level spells.
I understand that, for example, if you are a Wizard 2 and you also become Sorcerer 1, you might get access to level 2 spell slots (total spellcasting level = 3), but you can't select level 2 Sorcerer spells.
However, does that restriction also apply to spell replacement?
So, if I'm Wizard 1 / Sorcerer 1 and I take a second level in Sorcerer (which gives access to L2 spell slots), can I replace one of my L1 Sorcerer spells for a L2 Sorcerer spell?
I kinda expect a "Yes", but I just want to set things clear, because there's a chance for a "No"; you might interpret the second restriction as "only the spells you have" and the first restriction as "can't learn higher-level spells... but you may replace one of your lows with a high". It's a bit far-fetched, though.