Switching from AT&T post-paid plan to pre-paid: Is it possible, any gotchas?

[Bo]

Long time AT&T post-paid customer and the deal on the Best Buy iPhone SE for $149 (from this thread: http://forums.macresource.com/read.php?1...sg-2163311) has me considering going from post-paid to pre-paid. I don't keep up on this stuff but know that some here do. Also I haven't contacted AT&T yet but will to ask the same questions (Best buy rep didn't have many answers)

A few questions, all regarding AT&T specifically:

1. Would I be able to keep my number?
   
2. Can an off contract phone (my current iPhone 5) be used on a pre-paid plan?
   
3. Can a pre-paid phone be used on a post-paid plan if I decide to go back to post paid down the road?
   
4. In other words, what's to stop me from just buying the cheap SE and activating it on my post paid plan?
   
5. Presumably this iPhone SE will always be locked to AT&T...correct?
   FWIW I am out of contract on my post-paid plan and not part of a AT&T Next. I own my old iPhone 5 outright. I have no intent to leave AT&T for the foreseeable future. They've been very solid in my area.