Could Morgoth come back?

[Aiwendil]

Tolkien changed his view on the Dagor Dagorath theory in his later years

Very true.

and realised that there would probably never be a new contest between Morgoth and the Valar.

Not necessarily. There is no discussion of the Dagor Dagorath in later writings that is as thorough as that in QS, and there is no explicit statement that Morgoth would not return. There are several elliptical references to the Dagor Dagorath or the End, which are unfortunately ambiguous.