I once tried doing this (albeit for different reasons). As soon as I entered the new mobile #, Google Pay deleted all the bank accounts saved on the app until then and automatically started searching for all the bank accounts which have the newly entered mobile # as their registered mobile #. I found this workflow very disconcerting. Besides, this is totally not what I wanted. So, I rolled back my attempt to add the new mobile # and went back to my old mobile #.
This was a while ago. I also stopped using Google Pay several months ago. Maybe things have changed now.
Besides many modern day mobile apps and websites customize features for individual users or user segments e.g. I can top up my PayTM wallet with a credit card but many other PayTM users have said they can’t.
So it’s very difficult to say what will happen on your Google Pay app now if you try to add a new mobile #.
The best way to find out is to try it out. But I strongly recommend that you keep a backup plan in hand just in case things go belly up they way they did during my attempt.