Lord Gregory is a lovely example Birdseye. It's a great song that has many wonderful variations, versions, styles, not to name a multiple other differences that it can be used to interpreted it in so many different ways by so many people that you'd never find a best or 2 doing it the same (unless learned one from another & still they'd be differences over time) BUT when an experienced & well able singer has refined & applied their own chacteristics to all that it takes to sing the song then there would be no need or want for them to change it though they still may over time, after all it takes time to come to where one would finally settle with the way they want to sing it though again they it still may change even though they may not want to do it at the present.
The song may be heard the same with only a week between the hearing but I for one would expect that & if I enjoyed it enough the 1st time I guess I'd enjoy it the next just as well except that I would be expecting no change rather than the joy of listening to myself saying to "whoa, that was a nice turning of that phrasing" or the felling "that was a unique bit of ornamentation", you've already been surprised by the first rendition of the singer singing of that song.
I know that for me I stay pretty close to the way I'd sing a song but I know that it never stays the same for me & I don't intentionally try to keep it the same though I do sometimes intentionally try a change & over time, whither I sing it or not it changes. There are little inflictions that get added or get's dropped, bits of phrasing that might shift (I'm not talking about because of voice changes here as mentioned which is a completely different topic) emphasis may also be made in a phrase or elsewhere. I think for some the change is always happening for some the change is slower than what it is for others & for some it never changes & if they've got it to where changes won't enhance the song then the only reason they'd change it is so it may not sit stale for the singer not the listener but then that's a reason to change it too.

Barry