A few general rules:
Singing unaccompanied focuses the singer and audience's attention on the song, especially on its text. The highest praise any singer can receive is to be told, 'That was a great song'.
Practice, especially when singing unaccompanied, almost always results in improvement.
Singing it for years in a range of circumstances improves the relationship between singer and song.
When singing unaccompanied and solo you cannot sing out of tune - variation is the essence of traditional singing.

Actually, I've said this before:
The only rules are:
Sing
Sing only what you like
Permit yourself to alter what you do not like or do not understand
Allow other people the same right
Allow your songs to pass freely to others

If these rules are adhered to there is no threat to tradition - if I or anyone else mars a song with our changes, they will not survive, if I give someone a song and they make changes that are 'good', I get back more than I gave away.

And finally, don't worry about songs surviving. It is singing that preserves songs! SING!