Hi Brandon! These are great questions - I have to say, the primary reason we'd encourage a user to switch to a No Credit Check account is if they needed a family plan. In most other ways, Pay In Advance and No Credit Check are pretty similar. We're actually in the process of grandfathering the Simple Choice No Credit voice plans and moving to T-Mobile ONE No Credit instead, and I'm not sure if that would sway your decision one way or another. You can definitely convert your existing number from Pay In Advance, and you're also able to add optional features like Stateside International so that you can still call your friend in AU affordably. If you did choose to convert, you'd lose your existing Pay In Advance balance, so you'd want to wait until you've used almost all of it (but not quite all, because the line needs to be active) - and everyone does need to pay a deposit to activate a No Credit Check account. Whew!
Since we don't have live content on Support for T-Mobile ONE No Credit just yet, please let me know if you have any additional questions. I'd love to help you avoid a phone call if I can!