I'm sure there will be some that have to be replaced. Especially any that are probably around two years old or older. But they mentioned that they exelected to have some sort of replacement program or at least heavy discount for those that are forced into a new phone.
As with any corporate promise, I think people should still be prepared to buy new phones in the event T-Mobile doesn't update phones to support VoLTE.
Sure, those older phones will work with WCDMA fallback, but voice coverage won't be the same. 1900 GSM isn't as robust as 1x1900.