SIM Unlock S4 i337m or flash firmware? Changing Carriers

Hi all, I searched for a bit but couldn't find a direct answer to the question.

I bought a used Telus phone from a friend.
I have a Virgin mobile account and SIM.

If I want to put the phone on Virgin, can I just download and flash the firmware from sammobile.com for Virgin or will the phone still be on Telus?

Note, I did look into the frequencies, etc and all looks to be fine on the hardware front.

Thanks so very much.