Upgrading to Galaxy S7 on AT&T - how can I avoid AT&T's lockdown?

[AC Question]

I am an AT&T cellular customer and with my current Galaxy S5 I can't root. I'm worried that this will be the case when I upgrade to the Galaxy S7 in March. Do I have the option to buy another version of the Galaxy S7 (direct from Samsung, through other carriers like T-Mobile, or somewhere else) and then use my AT&T SIM card from my current phone? If not, should I buy a new Galaxy S7 from AT&T and just avoid all updates until someone figures out how to root? Any other thoughts or guidance are welcome! TIA!!