Per the Font of Magic feature, sorcerer can use Flexible Casting to create 5th-level spell slots at level 7, even though they are not typically available until level 9.
You can transform unexpended sorcery points into one spell slot as a bonus action on your turn. The Creating Spell Slots table shows the cost of creating a spell slot of [5th level is 7]
If such a sorcerer levels up while still having this 5th-level spell slot, can they choose a 5th-level spell as the spell they gain upon levelling up?
The Spellcasting feature states:
Additionally, when you gain a level in this class, you can choose one of the sorcerer spells you know and replace it with another spell from the sorcerer spell list, which also must be of a level for which you have spell slots.
Best Answer
Nothing in the standard rules seems to forbid it.
Flexible casting allows you to create spell slots:
These slots are treated no differently to the rules than normal spell slots.
The rules for learning spells say:
Since you have a 5th level slot, by a strict reading you could do this.
There are no other rules1 that I can find or think of that have any effect on the ability to do this, thus it seems to be permissible (or at least not explicitly excluded) by the rules.
It may not work depending on how the DM does leveling
There are no normal rules about when level up occurs, but many DMs allow leveling at the end of a long rest.
Since created spell slots vanish when you long rest, they would be gone before you could level and choose your spell.
If your table levels after a long rest, then this strategy won't work.
Almost certainly not intended (or at least planned for)
This is clearly a loophole use of the ability and is almost certainly not intended to work this way.
For example the following sentence of the rules about learning new spells says:
This is referring to the spellcasting table for Sorcerer which outlines the slots a Sorcerer has at each level (and number of spells known). Note that there is no qualification here (eg "...unless some other feature grants you more slots"). The implication in the rules, though it is not explicitly stated, is that when the rules say "level for which you have slots" they mean "level for which you have slots according to the table".
Probably not worth it
A DM can certainly allow this and I don't think it would destroy their game alone (though I have not and likely will not ever play with this ruling). The exploit only seems to exist for 5th level spells so the impact, at least, is contained to that alone. In the end the sorcerer is trading almost all of their sorcery points just for the chance to cast up to two 5th level spells one level earlier. Even so, I would think hard before allowing it unless you think this will enhance the fun at your table.