It might be that they automatically fill the address on their web browser with the 'old' (previous church) URL address.
For instance, our church's Parish Realm URL is https://onrealm.org/75StsCnH (you can find yours in the Admin tab, Manage Realm Account, Account Summary).
I don't know how to change that in the Connect app, though. Generally, I would contact Support, they are extremely helpful!