The Vienna Boys Choir is:

[GrizzRich]

I’ve used the term revolving cast or revolving membership to describe such a body.

[HeyHomie]

@chrisk

So, in keeping with that, we could coin the term that the Vienna Boys Choir is a Thesean group.

We have a winner! I’ll be telephoning my contacts at the OED.

[Big_Norse]

@AskNott

It’s like what Lao Tsu said about the river. If you dip your toes in the river, then do it again twenty minutes later, in one sense it’s the same river, with you on the same bank. In another sense, the water you first dipped your toes in is far downstream, so it’s not the same river at all.

Or as someone else (I have no idea who) said: “You can never cross the same river twice.”

[Princhester]

I’m not sure we have quite the audience of Dan Savage, but nonetheless I think we should do our best to get this term off the ground.

“Thesean. adj. (of an object or group) Persisting in identity despite replacement of some or all constituent parts or personnel”

I consider it the absolute duty of all dopers to use this term wherever possible, and to explain the meaning to anyone who asks and will listen.

[DaveRaver]

Interchangable.

[Johanna]

@Cub Mistress

Ok, mutable

Menudable.