At first, VoLTE will be used to deliver "Rich Communications Services" (which look like something similar to what Voice over IP/Video over IP outfits offer today) rather than completely replacing narrowband voice. It will be awhile before Verizon is willing to commit to that, due in part to LTE outages in late 2011.
On the flip side is MetroPC, which has pushed VoLTE as hard as it could.
Then there's Sprint, which will stick to voice over 1x for the foreseeable future, but routes Direct Connect over IP. Sprint's latest DC incarnation seems quite similar to one facet of RCS over LTE.
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.