Well I don't know what the difference between a phenotype and appearance is. In the world of biology (where the term phenotype came from) they pretty much mean the exact same thing.
We have an appearance changer on our world (or phenotype changer.) It determines the PC's size and speed and only allows them to have the option of changing to other appearances/phenotypes that have the same parameters.
Installing this version of CEP (or 2.6 for that matter) completely breaks that system. When the dialog comes up all the options are dysfunctional. So this is why I opted not to go with CEP 2.6(+) on our world, because this was a very popular feature for our players. But I still encouraged folks to go ahead as players and update to 2.61.
Until it turns out one player who did, could no longer log in one of their toons (their main toon in fact.)
Once we narrowed down the problem, and I suggested her reverting back to CEP 2.3 or 2.4, bam the problem was fixed.
I would call that not backwards compatible if you were to ask me.
The problem might not be that big. This is what the player reported doing, rather than a complete revert:
"In the end I simply replaced the 2.6 add phenos Haks with the 2.4, and it worked"
Sounds like the problem is isolated to this. But having to piecemeal together a combination of different CEP 2.0+ versions, seem not right to me.
And again I am not trying to come off as an ungrateful azzhole for the work that was put into this. I know this must have been a lot of work, and I wish we could be one of the worlds to update to this. But, damn we can't have this update breaking things for us, and it's sad because CEP was supposed to be a universal hak pack that multiple worlds could all use, so that players could play them all. And after Aventia using CEP through multiple generations of CEP, and owners, to have the currnent version, not only break a system in our world when installing it, but also make it so those players who update, might have problems like this seems like a big ole slap in the face for a world that has supported, promoted, and championed CEP for what? A decade?