It says right in the article:
"T-Mobile’s CTO, Neville Ray says that the key to the success of both these companies is their path pursuing LTE
. The next step will be that T-Mobile will migrate MetroPCS customers to equipment that use T-Mobile’s network."
"From day one, T-Mobile expects to start selling phones that have the MetroPCS brand, but use T-Mobile’s network.
After migrating existing subscribers off of MetroPCS’s CDMA network, it will be shut down and the spectrum will be used to deploy LTE service. The process may take a couple years to complete."
I was in a local metro store buying my Esteem last October when a customer came in with an ancient flip phone looking for a battery.
That is the type of customer they'll have to convert to LTE.