Default Advice needed on best option for unlocking or not

Hi guys n girls ok just an update previously I had s4 fully rooted and sim unlocked on 4.2 on mdc that phone has now got a broken LCD and digitiser I have now got another sprint s4 perfect screen and locked on sprint which has 4.3 and is also on mdc software version and baseband mdc build number is mk2 I would like this sim unlocked is that possible as I don't understand this stuff about Knox or would you say that I just swap the screens around and stay on my original 4.2 which I downgraded to the mdc ? Please help