Inspired by this question about a Simulacrum copy using a spellbook, I started wondering how locked into their initial state a Simulacrum copy would be (when I was wondering exactly how useful a spellbook would be to the simulacrum). The text of the Simulacrum spell states (PHB, p. 276, bold added):
The simulacrum lacks the ability to learn or become more powerful, so it never increases its level or other abilities, nor can it regain expended spell slots. If the simulacrum is damaged, you can repair it in an alchemical laboratory, using rare herbs and minerals worth 100 gp per hit point it regains.
Now, this got me wondering: what if a high level Cleric (used as an example to sidestep the wizard's need for a spellbook) were to be copied via the Simulacrum spell? On the one hand, that character can't "learn or become more powerful," so that would imply that their mental state is somewhat locked in: and besides, there are two major benefits of a long rest (regaining hit points and spell slots) that they are specificaly denied. On the other hand, when you prepare a new list of spells as a Cleric, you don't necessarily become "more powerful," or "learn" new information: it's more of a lateral move, similar to dropping a sword and drawing a bow instead. You aren't necessarily "more" or "less" powerful, but rather are better suited to your current challenge.
I also got confused because the rules on a wizard copying their own spellbook state (PHB, p. 114, bold added):
You can copy a spell from your own spellbook into another book... This is just like copying a new spell into your spellbook, but faster and easier since you understand your own notation and already know how to cast the spell.
The bold section gave me pause, because it is referring to any spell in the book, not just those spells the wizard has prepared. This implies that "knowing how to cast the spell" is distinct from having that spell prepared, which makes me question whether preparing spells falls under the "ability to learn."
My instinct is that a simulacrum can't prepare new spells, because their inability to natively regain hit points or spell slots makes me think of them as basically being restricted to the resources the creature would normally have between long rests. But I'm sufficiently unsure that I'm asking it here. Can a simulacrum of a caster that can select different spells to prepare at the end of a long rest do so, or are they locked into the spells that were prepared when they were created?