Quickly looking at their website, it seems that they transparently provide all avatars from Gravatar as well, so the migration seems to be pretty much painless. Basic replacement to use their server is just matter of changing base URLs, however to support federated behavior, you have to install pyLibravatar. Weblate in Git now supports both these ways.
While implementing the client side, I did also setup my own instance of Surrogator to provide avatars for some of my domains. Surprisingly this worked fine without problems, but let's see how much used this will actually become.