A wizard casts Tenser's Floating Disk, creating a:
horizontal plane of force, 3 feet in diameter and 1 inch thick, that floats 3 feet above the ground
The movement of the disk obeys certain rules:
If you move more than 20 feet away from it, the disk follows you so that it remains within 20 feet of you. It can move across uneven terrain, up or down stairs, slopes and the like, but it can’t cross an elevation change of 10 feet or more. For example, the disk can’t move across a 10-foot-deep pit, nor could it leave such a pit if it was created at the bottom.
If the wizard tried to cast Levitate on Tenser's Floating Disk, how does the spell affect the disk's movement?
I can think of a view potentially valid interpretations:
The spell fails for lack of a valid target. A "plane of force" might not count as an "object" for the purpose of casting Levitate, in which case it's not a valid target:
object of your choice that you can see within range rises vertically, up to 20 feet
This answer by David Coffron to a similar question suggests that the disk is absolutely not an object. That sounds like a valid ruling, but to my mind, whether a magical creation counts as an "object" is subject to DM fiat.
The disk doesn't rise. Perhaps the disk is magically prevented from rising into the air, because it:
can’t cross an elevation change of 10 feet or more
One might also think that the disk cannot rise if it is within 20 feet of its creator, since:
The disk is immobile while you are within 20 feet of it
This seems like a weaker objection, since someone might cast Levitate while the disk is 20 feet or more from its creator.
Once aloft, the disk doesn't move. Normally the disk magically floats toward the caster, but once it's subject to the Levitate spell it can only move by interacting with its environment, which the limbless disk presumably can't do:
The target can move only by pushing or pulling against a fixed object or surface within reach
A good analogy here would be a flying creature targeted by Levitate, on which interpretations appear to vary.
Once aloft, the disk moves at its new height. Let's say the DM rules that the disk is a valid target. Let's also say that the disk can be made to rise, because the rule only limits the disk's own self-driven movement. Finally, let's say that the magical propulsion of the disk overrides the movement limitation imposed by Levitate. In this case, Levitate has simply changed the "ground level" for the disk, which now floats at the new level.
My initial intuition was that (4) was right and Levitate could be used to "unlock" Tenser's Floating Disk. After further consideration, I suspect that (3) is the correct answer (provided the DM rules that the disk can be targeted, overcoming (1)).
Which answer do others prefer? Is there something I've missed? Will a DM who allows (4) via the "Rule of Cool" quickly come to regret it? I'm happy with both RAW and RAI answers.