The DC to dispel true polymorph with Dispel Magic using a 3rd level slot is 19, I believe.
Dispel Magic using a 9th level slot automatically succeeds against True Polymorph.
Does the DC for a successful Dispel Magic decrease from 19 by using a higher level slot that is not 9th?
Best Answer
The DC to dispel true polymorph is always 19
Dispel magic says:
The only effect1 of upcasting dispel magic is that dispel magic can automatically end spells of a lower or equal level to the slot used:
Upcasting dispel magic has no effect on the spell DC. Only on the level of spell it can automatically dispel.
The DC only ever depends on the level of the spell you are trying to dispel.
1 - The only effect relevant to this discussion anyways. Of course it has side effects like changing how difficult it is to counterspell for example. (thanks @Slagmoth)