Does a spell persist if it is cast on an object, and the object changes into a creature? For example, spells like Light or Darkness, are linked to a movable object but can't be cast directly onto a creature.
Thus if, through magic, the object becomes a creature, does the original cast spell still move with the creature, does it stay still at the point where the object stopped being an object, or does the spell stop?
The rules only specify what the conditions for targeting are when the spell is cast and do not cover cases where that condition changes after the fact.
The rules assume that the target is decided when the spell is cast and that the validity of that target does not change after that. However, there are obviously cases where that can occur (as pointed out by your question).
As always in 5e, when the rules aren't clear or are silent, the DM must step in to make rulings to fill the gaps.
As a DM, it makes complete sense to allow the spell to be suppressed or dispelled once the target becomes invalid. However, since there a quite a variety of spells and targets it may make sense to evaluate each one on a case-by-case basis as they come up. This is not a case that happens very frequently at my table or any I've played at, but if it becomes enough of an issue, a DM may need to make a general ruling to keep the game running smoothly.
But, again, as this is not spelled out in the rules anywhere, it will have to be a DM call to decide what happens.
Jeremy Crawford states much the same in an opinion written on Twitter:
There's no rule governing what happens when a valid spell target temporarily becomes an invalid target. A good rule of thumb is that the spell is suppressed while the target is invalid.
For Light/Darkness specifically, I would rule that the effect gets suppressed for as long as the lit thing is an invalid target. If they become a valid target within that time again, then they begin to light up again.
It doesn't make sense for the Light/Darkness to become detached from their original targets nor does it seem like the spell would necessarily be dispelled. Thus it seems like a very easy, and workable medium option.
I've ruled this way for similar spells a few times at my table and it made sense and it worked well and we really didn't think too hard about it honestly. I can see other ways of ruling potentially working as well, but this is the way I've done it.
This is all DM ruling area.
If the spell necessarily is cast on the object, i.e. you cannot cast the spell without an object as the target, then the spell should stop operating for as long as its target is not an eligible target. If the target turns back into an object before the spell's duration runs out, the spell may resume effect.
If the spell may be cast on an object, but doesn't have to be, e.g. Darkness, then the DM will need to rule whether the spell stops operating, or whether the spell disconnects from the now not-eligible target, and remains stationary at that location.
Seeing as how the rules aren't specific and it's up to the DM I'd go for a roll off. Make whatever favours the most interesting scenario result more likely. Fate favours this works or not. Of course you could roll a dice then say what happened is what you think is more interesting anyhow.
I prefer a focus on the story. Assuming there is any.
The creature is now an ineligible target for a spell that targets an object so the spell ends.
