Your top priority should be "Are my backups good? / Can I trivially roll back any breaking changes?" If an account oopsie can permanently compromise your users' photos, then you have bigger problems to worry about.
But assuming your backups are good, there isn't much to worry about. I recommend you don't take my word for it and thoroughly read the documentation of each of the services you want to put behind Authentik, but in general, when a service is configured to use SSO, if a user with the same ID already exists on the target service, they are simply merged. The most common ID for this is the email associated with that user on both Authentik and the service. Worst case, if the ID doesn't match, you either get an error saying the user is invalid or you get a new user created on the target service.
As another Brazilian, good riddance to that crap. The current standard is so much better. Anything that is compatible with the American standard is automatically a mistake. Now if only we could standardise the whole country on 220V and drop the need for different 10A/20A sockets...