Default Questions about "flashed" phone!

My mother made me flash my phone to cricket because they have 5 phones for 100 a month. They told me not to factory reset the phone, call *287 or update it. I was wondering where is the "flashed" data located on the phone? Can I backup the flash files then after an update/kernel patch that I can reapply them. Also, will installing a custom kernel or rom break the flash? Sorry for all these random questions, but I don't wanna **** up my phone.