T-Mobile said last month that it had three major markets and 500,000 customers still on the legacy MetroPCS CDMA network, and it looks like T-Mo plans to shut ‘em all down in the next month and a half.

A MetroPCS support page says that phones operating on the legacy network — its CDMA network — “must be upgraded by 6/21/15.” Metro goes on to say that after that date, it’ll be converting its legacy CDMA network to its new GSM network, and any CDMA customers that don’t upgrade “are not guaranteed to have service” after June 21.

T-Mobile has been refarming MetroPCS’s CDMA spectrum in order to improve its own GSM network, and in the process its been urging CDMA MetroPCS customers to upgrade to a GSM phone by offering trade-in credits and other discounts. That’s the case now, with MetroPCS offering subscribers with a CDMA phone an instant upgrade credit for trading in their old phone that can be used that same day. Expect that upgrade push to continue in the coming weeks as T-Mobile works to totally sunset the MetroPCS CDMA network and beef up its GSM-based service in the process.