Default [Q] Verizon G2 with AT&T Sim card help!

Hey all. Sooo I recently obtained an unlocked verizon G2 and am using my ATT sim card in it. I absolutely hate it, primarily because I miss my 4G LTE. So I am supposed to be trading for an S4 tomorrow and just wanted to clarify something:

The guy that gave me the phone said to not factory reset it or it will need a Verizon sim card to boot up. Is this true? This is my first experience with an unlocked phone.

If it is true, is there an alternate way I can erase all my content before trading this phone away that doesn't involve a factory reset?