This would need a custom texturepack added to the server, and have the plugin add NBT tags or something to the wolves to specify the skins for the wolves.

Not sure if that's even possible, worth looking into.

Mojang implemented this 👍

I don't think I would like to implement the option to change the type of wolf you have - but actually, maybe it would be nice to keep all your current "stale wolves" - so maybe either:

1. change stale wolves randomly on first encounter on new update
2. make option to change wolf type with a command