If you have your changed phone number, simply open Truecaller and you shall be prompted to move it right away.
Note, make sure to be signed in to the same Google account you purchased Premium on.
If Premium is still not activated tap on "Upgrade Premium" in the menu, restart the phone and try again.
Exception to above:
- Due to platform differences, it is not possible to transfer subscriptions from (Android to iOS or vice-versa)
- For Web purchase: Subscription transfer on any platform is not possible.