Sign in With Apple – New accounts only?

Hello everyone

We are preparing an update to our app and I have questions regarding Sign-In With Apple.

In older versions of our app we allowed users to sign up to our service with their LinkedIn or Xing account while also offering our own sign-up that doesn't require any socials.

We can remotely control our Onboarding flow, so we have disabled creating new accounts with either of the two options (Xing was removed a while ago after they ceased offering their public API).

What I was wondering now is that if we have to offer Sign-In With Apple if the branch of our Onboarding for existing users still offers the options to log in using Xing and or Linkedin (Xing resets the password and changes their profile from a Xing one to one of ours, we are thinking of doing the same for LinkedIn).

If that is the case that would involve a lot of work for us, and in the current situation we are a bit strained in regards to resources, as well as the situation that I'll be leaving the company at the end of November.

I hope some of you have either had experiences with the same kind of offering, or if someone from Apple could give an answer here.

Also, as an off question.. Sign in With Apple is an iOS 13 feature, so how should this be done on earlier versions of iOS? Would it be OK to not offer it there?