E-Mail confirmation on registration but not logon

I have a website which has a lot of users before implementing e-mail verification. I use ASP.NET Core 1.1 I want new users e-mails to be verified but not previously registered members.... So now if I add this line to Startup.cs:

options.SignIn.RequireConfirmedEmail = true;

previously registered users will need to have their e-mails validated and this is not practical in my site... If I set it to false then new users will receive an error if they open ConfirmEmail method... so I need a solution to be able to have it set to true but previously registered users are able to login without confirmation.

Thanks a lot