I think we've been here 'afore. Trad to me is a song/tune/dance that is carried down through the generations. For old stuff, many of the original writers/creators of the piece are long forgotten but with modern recording equipment (written or audio/visual), and of course copywrite rules etc, this is less likely to happen. So i reckon a song like Fiddler's Green by John Conolly could now be regarded as 'traditional' as it is sung by many in England and abroad and occasionally altered a bit (as John has acknowledged). Same with many of Ewan McColl's songs and even Bob Dylan's. Of course this could be said of Lennon & McCartney and lots of less 'folkie' (whatever that means - whoops that's another thread i think) writers.

I don't think altering a song/tune makes it less traditional, more like 'variation of theme by......(as in classical music)'. Or even a song adapted from an original etc.