How do I rearrange my watch faces in watchOS 2?
AFAIK, there is no way to easily rearrange them. As you probably know, you could take note of which complication went where on every and delete all the watch faces and then add them again in the order that you want, but that would be very time consuming and if you change your mind later then you'd have to do the whole thing over again. Apple should have let the user rearrange watch faces in watchOS 1 or at least watchOS 2.0, but it looks like we'll just have to hope that this is implemented in watchOS 2.1 or 3.

Adding on, you can send feedback to Apple here.