Frequency band mofications for Samsung Galaxy S4

I recently purchased a used, unlocked Samsung Galaxy S4 SGH-I337M. The phones native bands don't include the 1700/2100 AWS required by the cell phone provider (WIND mobile). However, the phone is able to get both talk and data as desired. The previous owner told me that he modified the phone to allow these bands.

I'd like to know

1. How this was done?


2. Whether there is a chance a firmware update or factory reset will undo this feature?