I have a homeserver that is using LDAP and I want to migrate to OpenID Connect. This requires that existing users can login with OpenID Connect. If an existing user tries to login, I get the following error and login doesn't work: synapse.handlers.oidc_handler.MappingException: mxid '@snowball:test.example.com' is already taken

I think this behavior is reasonable as a default because merging with existing users may be a security problem if the admin isn't careful.

This patch adds a config option to allow using existing users. This seems to work for me but needs more testing.

What do you think? Is this a good solution? Are there any better ways to migrate from LDAP (or other password login) to OIDC?