For example. a Spirit Naga is a "10th level spellcaster", and "has the following wizard spells prepared". The text explains that the DM may substitute other spells for the ones given, but it is not clear whether such substitutions are permanent for that individual or simply represent the creature's selections from a spellbook (like a wizard) or other mechanic. And it is not stated whether slots are regained after a short or long rest – if ever!
So, if the PCs engage a Spirit Naga and force it to retreat to a safe haven, what will the Naga's spell repertoire look like in two hours? In a day?