One of the reasons for lyric drift is that some of the words are no longer PC This usually relates to the negro songs by the likes of Stephen Foster etc. Also words that meant one thing at the time of writing have changed their meaning today an example is in "The Old Dunn Cow"
Just then there was such an awful crash
Half the bloody roof gave way
We were drowned in the fireman's hose
but still we were all GAY
Unfortunatly the more we find new meanings for common words the less songs we will be able to sing in future
Bill