Officially supported, seamless transition. You don't have to upgrade until the plugin is ready. I'm not 100% sure what the problem is? As I said earlier, aside from actually getting this done which we plan to do soon, what else is the problem that we should look into?
I'm happy to wait, that's not a big deal. Although I think there should be heavy visibility into loss of core functionality changes - I'd have had no idea that things were expected to break had Julian not IM'd me to stop me from upgrading voluntarily (he knew that we relied on the Gravatars from the previous conversation where I had requested that this functionality not be removed.)
What we have today is that users sign up and get Gravatars automatically. No extra work. We have local avatars disabled as we do not want a mix of things and we do not want to be serving out images from our NodeBB servers. In a seamless transition we would like it if the users do not lose access to the Gravatars that they have now and that new users continue to have Gravatars by default without having any idea that they need to disable them, upload anything, etc.
There are some simple upgrades that we'd love to see... like removing the interface to upload an avatar when the functionality is removed (currently they are confused because they click it and it doesn't work) and instead having a link that takes them to Gravatar instead. But that is all extra and pretty trivial.
What we don't want to see is current working Gravatars lost and/or new users not having Gravatar work transparently without needing to do anything special.