iPhone 5 At&T Flashing question

[iAreAirwick]

So since I have an unlocked AT&T iPhone, would I have to get a new plan or anything for the iPhone 5 once the new tool comes out? Or do I have to change my current plan.
My current plan is 100$ for 4 phones and I'm giving my little brother my iPhone 4 flashed by you guys and my older brother and I are getting iPhone 5's so we can't really change the plan. Would we be able to keep the plan and just insert the sim since it's unlocked?


Edit: I read your comment about the differences between the iPhones and I don't mind using the Fallback option on the iPhone 5! Thank you for the information, however I'm still wondering if the unlocked AT&T iPhone would work. Would it have to be factory unlocked? Or will I be able to unlock it from a unlocking website and still use it? Please answer the questions above as well.
Sorry for being a bother.