Hello Annadai,
Thanks for reaching out to the Apple Support Communities. We understand you are experiencing some issues with trying to download or update apps on your iPhone after changing your region. We are happy to help look into this with you.
To clarify, you are attempting to download new apps that you haven't had on your iPhone in the past, or are you attempting to update apps that you've had? Have you tried any troubleshooting steps yet to try to resolve this issue?
Have you tried signing out of the iTunes and App Store and signing back in?
Keep us updated on how it goes.
Best Regards.