To which I'd add non-deliberate changes because you misremember, or where there's an instinctive change to a word combination that comes easier off the tongue.

Which is why a change from "I never" to "I ever" - which is much harder to sing - is a strange one. I'd say it must have come from a singer learning the song from a written text which had it down as "I ever", and who had an excessive regard for the written words, which stopped them from instinctively changing it to the more natural "I never".

I'd disagree about the difference of meaning, because they both mean it's the first time the singer has ever dreamed that dream, though "never" does perhaps emphasise that uniqueness a little.   

I can see why Ed McCurdy would have been irritated at the change. It's clumsy - grammar as such ain't that important, but when you've said "the strangest dream I've ever dreamed" you've finished the sentence, and it sounds as though "before" is just stuck in to make it scan. That doesn't apply when it's "never" - and in addition "never" indicates a slight pause after dream which the tune needs, and which doesn't really belong in the "ever" version.

Last night I dreamed the strangest dream, I've never dreamed before.

Last night I dreamed the strangest dream I've ever dreamed before.

Tiny changes can make quite a difference.