It makes me wonder if you would be able to take your current phone and have a new sim card put in that would allow you to use all frequencies. In the case of the Sprint version of the i6, all the frequencies of the other carriers are built into the phone currently.
Per Apple's website:
Model A1586* Model A1524*
CDMA EV-DO Rev. A and Rev. B (800, 1700/2100, 1900, 2100 MHz)
UMTS/HSPA+/DC-HSDPA (850, 900, 1700/2100, 1900, 2100 MHz)
TD-SCDMA 1900 (F), 2000 (A)
GSM/EDGE (850, 900, 1800, 1900 MHz)
FDD-LTE (Bands 1, 2, 3, 4, 5, 7, 8, 13, 17, 18, 19, 20, 25, 26, 28, 29)
TD-LTE (Bands 38, 39, 40, 41)