Move a post-paid AT&T NBI phone number to a pre-paid account?

I want to move a post-paid AT&T NBI phone number to a pre-paid account.

My old employer initially paid for my cell service, I owned the phone.

After they discontinued that perk, I assumed the payments and kept the number.

I spent two hours at the AT&T store and another two hours on five different AT&T support calls, all to no avail!

How do I do this?