Yes, the tunes change just like the words. I believe the change is more inadvertant with the tunes than with the lyrics. It "just happens" naturally with the tunes---as a result of folks making changes based on their talent and creative abilities, and, also, the musical emotions of the moment. The tales told in the songs can stay close to the older version, based on the memory of the singer and/or the collector. BUT people might want to personalize the words to a song---a more local geographical reference, or by putting in a better known person or two!

How about retired cowboy Del Bray's version of Barbara Allen. I met Del in a Cheyenne, Wyoming bar back in 1962. We shared a six-pack in my hotel room across the street while swapping some songs. He sang:

In Medicine Bow where I was born,
There was a fair maid dwellin',
Made all the boys ride saddle sore,
And her name was Barbara Allen.

The tune was pretty much one of those that were often always used for this old ballad---or, at least, that was how I remembered his tune when I set out to record it. I wrote down the words while he was singing it. Did I get 'em right? I hope so! (I think it is in the D.T.)

In my version of "The Lakes Of Ponchartrain", I admit to inserting the date of my own wedding anniversary into it. ;-) And I sang it to the old tune of "Come All You Tramps And Hawkers" just because I loved the tune more than the one Helene Stratman Thomas had collected in Wisconsin for the Archive Of Folksong at the Library Of Congress.

It was on the third of January I bid Cairo town adieu,
Traveled down the river road my fortune to pursue,
No money in my pockets, no credit could I gain,
'Twas then I turned with longing to the lakes of Ponchartrain.

I swung on board of an old boxcar just as the day did dawn,
I rode the rods from sun to sun---then I lit down again,
As the shades of evening fell, the low ground I did gain,
And there I met the creole girl by the shores of Ponchartrain.

Keep in mind, this is just one guys point of view. Could be an intersting thread...

Art Thieme