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.
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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.
Best Answer
Your sorcerer cannot swap in a 2nd level spell
As you have noted, multiclassing rules state that:
When you are replacing spells, you are determining what spell you no longer know, and what spell you do. That is, put more generally, you are determining what spells you know from one of your classes. So, the multiclassing rules apply.
In your example, you determine what spells you know as if you were only a level 2 sorcerer and nothing else. A pure level 2 sorcerer has no 2nd-level spell slots, so they cannot not replace a known spell for a 2nd-level spell at level up. And by following the multiclass rules, that tells us that a multiclassed level 2 sorcerer also cannot replace one of their known sorcerer spells with a 2nd-level spell either.