There are two types of change here - one is where the words are changed so that people listening will understand the song with the original meaning, because the word used has changed its meaning - and the other is where there's an attempt to make sense of the words that changes the original meaning. Mondegreens fall into that category.

An odd little example is in Eric Bogle's No Man's Land - as he wrote it he sings "I've been walking all day" (at least I think he does) - but when sung in Ireland that is often understood (and printed, for example in the Ossian vol.3) as "working all day".

As for "gay" (eg in The Old Dun Cow), that word hasn't lost its traditional meaning, it's just got another meaning added in alongside for the time being. I suspect that the earlier meaning will reassert itself before all that long.