Default c6616 problems and questions

Hi, I just recently switched to the gnex. to the xperia z, and the last xperia i had was the x10. I vaguly remember flashtool and what not. but the weird part is I have the C6616.. so idk how to upgrade it and be able to flash new roms. I have a locked bootloader and it can't be unlocked. I'm on 4.1.2 10.1.1.A.1.319 rn, is there anyway to update, root again, and then flash a rom (trying to get http://forum.xda-developers.com/xper...-10-4-t2349656 )
can I just flash any ftf or does it have to be for c6616.. can it be c6603?