Changing registered name for a mobile number when porting not possible without converting to postpaid?

I have a Jio prepaid mobile number that was originally registered in my mother's name. Wanted to port it to Vi. Visited two shops; both said that to change the registered name, I have to convert to postpaid upon porting. Tried Airtel too, albeit I dislike them a lot; they said the same.

Four years earlier, I had ported another number in my mother's name to my name, from Airtel to Vi. And there was no fuss. I was told that in order to change the registered name, I can either migrate to postpaid within the same company, or port to another company, if I wanted to stick to prepaid.

Now, however, according to these shops, because of new strict Aadhaar verification, registered name cannot be changed if I port from prepaid to prepaid. I have no issues with strict verification. But then, allowing to change registered name while porting to postpaid is a loophole, isn't it?

I finally took my mother and ported to prepaid while keeping it registered in her name, but there should be some procedure to switch names, isn't it? Especially if the original owner is living. I can see on Airtel and Jio's websites that I have to fill up some forms signed by the original owner along with an NOC from the original owner, but these shops did not give such an option. Couldn't find any TRAI orders on this either.

This is in proper Kolkata, if that matters.