I just managed to find a workaround for my case at least.. my home store is uk but it has changed to USA and I couldn’t change back even though subscription cancelled for Apple Music as needed to wait until November 24 2018 (today is 10th November )
1) Changed my password for iTunes using Apple.com
2) Signed out of iTunes on main phone
3)Signed in on another device (iPad) and then went to app stored and got the message “ this Apple ID is only valid for purchases in the UK store. You will be switched to that store” and pressed OK
4) went back to phone and App Store
5) singed in again with new password
6) the same message as in step 4) appeared; selected OK and then the store changed
Didn’t need to wait for Apple Music to expire - which is due on April 24
So perhaps just changing password and signing out and in again is good enough. You need to select an App then from the store that is the one you want to change back - eg I had set my phone to US by mistake before so chose An app to update that I knew I had downloaded from uk App Store previously to force the message shown in step 4/6
Good luck!