Over tha last half year I have returned to singing regularly-ish after thirty years absence - interesting.
I find myself remembering songs fairly quickly, sometimes with a little effort, but when I check with the original texts (kept most of them on file) I find I have made changes which have established themselves without altering the sense of the songs.
I watched McColl regularly towards the end of his life and I noticed his memory beginning to go slightly, yet he never dried up or hesitated - not while I was there anyway - he always managed to put something in which fitted perfectly; you wouldn't know if you weren't very familiar with his repertoire. I reckon this was because he was so steeped in the poetical form of the songs that he could re-create when necessary - he was singing the story, not just the words.
I've come to the conclusion that this is the secret of all (certainly narrative) singing.
Jim Carroll