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

[bobinmurphy]

I believe the FCC rules for unlocking a phone are:

For pre-paid your account must be current and at least six months of service

For post-paid your account must be current and at least 40 days of service

I've used this to purchase GSM phones on T-Mobile, use them long enough to qualify for an unlock, then unlocked them and moved them to AT&T.