"Dreamt" and "dreamed" are a curious pair. They mean the same, and both seem to get used equally in my experience, but they aren't always interchangeable. It'd feel completely wrong to sing "I dreamed I dwelt in marble halls" or "I dreamt a dream in times gone by".

You can even have both variants in the same song, but even though it wouldn't change the scansion it would't work to switch them round.