If I flash an AT&T ROM to a...?

[Jembo]

If I flash an old unlocked Galaxy S3 that was originally tied to a Canadian carrier with an AT&T ROM for the same phone, will that f*ck anything up.

It seems that AT&T is the only carrier to have ported Android 5 to the S3.

 

[isekii]

why not go for custom roms instead?
There has to be so much dev work done on the S3 that you'd have your pickings.

 

[paperwastage]

it's possible to mess something up

try searching xda forums for your specific model number? they do a lot of tinkering (but you need to be comfortable with flashing ROMs/unlocking bootloader if possible/commandline/... )

 

[Jembo]

Yeah, I messed around with CyanogenMod & some others but I get tired of some things working & others not. Also, I don't have internet at home so all this has to be done at a library, therefore I don't always have time to dick around with different ROMs.

So, as for flashing, rooting, etc., not a concern for me. I just want to know if this can,
A-mess up my unlock(I paid for this)?
B-brick the device?

 

[paperwastage]

A) shouldn't mess up your SIM unlock (backup your EFS before you do any flashing. if you wipe your EFS partition for some reason and have no backup, phone might not work)

B) possible

as I said, post your model number and search for it on xda

 

[core2slow]

Didn't see you mention anything about boot loader, so yes, you will mess up the phone.