Default Got some troubles with S4 sprint's Plan

So I bought this phone from a user on ebay, he's not using Sprint anymore so he sold it, and i'm not even in US so I just get my phone unlocked by Downgrade modem to MJA method.
But now i want a Kitkat upgrade so I got some question:
- Does my phone get sim locked If I upgrade OTA? My modem is MJA atm. After few hours reading and searching, I found that this method can;t use if above MK2.
- The owner of this phone said he already pay over 600$ for Sprint and extra fee to terminal his account. I checked my HEX number and it said the phone got clean ESN, so does it still get unlocked officially by Sprint?