If I was to jailbreak my at&t iphone?

If I was to jailbreak my at&t iPhone and change it to a t-mobile phone. Will At&t find out and charge me the cancellation fee?

Lol no. That simply does not happen.

Only if you are currently in contract with AT&T will you be charged the fee.

They will not charge you anything if the phone isn't on contract. It's legal to unlock a phone if it's not made in 2013, so why would AT&T be mean about it?