Samsung Galaxy S5 — samsung s5 sm-g900f repair

My phone galaxy s5 sm-g900f previous owner said that, phone was unlocked by using Samsung box (cable unlock) in local service centre.
they changed imei and s/n. s/n is 00000000000 and imei is not equal to battery back imei .
and changed csc code o2u to xec.
present my phone working well and has not any problem.
my questions are
1) Can i upgrade to lolipop G900FXXU1BOC7_G900FXEC1BOC1_XEC
2) has any problem when i upgrade to lolipop, i mean is it again relock or network problem?
3) can i change xec to o2u (G900FXXU1BOC7_G900FO2U1BOC1_O2U) , i mean making branded phone.
4) can i install any firmware( xec,o2u,btu,vir) belongs to sm-g900f? ( i want install non network firmware btu)

my phone model no and serial number entered in kies3/smart switches ,they shows
Initialized version: PDA:0C7 / CSC:0C1 /PHONE: OC7 (O2U)

my questions are

4) what happened when i upgrade firmware and initialisation using kies3/smart switches? is it restore original imei and s/n and also relock again?

5) can i restore original imei and s/n using samsung box? what happens when i restore original imei and s/n using Samsung box?

plz tell me and give best suggestion? .