If you can only do this on long rests it basically doesn't affect balance, right? You don't ever get extra resources, just different ones.

We let somebody do this in our 4e campaign and it was fun. They had like 4 different character sheets that all represented the same character, they just picked one for each adventure. It let us vary our party composition without having to introduce new characters to the narrative.

(The conceit was that they were experimented on by a mad mage who imbued them with elemental essences in a failed attempt to create a pan-elemental being; instead they manifested one element at a time, and each of their forms had its own abilities. Mechanically their race was genasi, with a different elemental aspect for each class.)