I switched away from Google and haven't had a single issue going from 'login with google' to a regular email/password FWIW. Every site has offered some kind of painless recovery.
This is a bit surprising to me because I switched emails some time ago, and most sites were painless, but there were a fairly long tail of sites that had no self-server mechanism to change email from one to another. A fair number of them required phoning in to read off my new email to a CSR rep. Another pile of them had no mechanism to change email at all, their only solution was to open a new account.