it seems as if Coach was checked in full. only FC seats were available.

The agent should have explained to you that Coach was full and that they'd put you on the standby list. Then, as elite Coach pax got cleared into F, you'd be cleared into Coach provided there were not other standby's ahead of you. The agent went against the rules and forced you into F. lucky you.

The only other thing i can think of was that the agent reassigned you on the new flight and somehow assigned you to Y class (not cabin, but fare class) which in turn allowed them to seat you in F. This has happened to me once.

check your OP statement. Id bet that the cle-iah segment you actually flew will show either AU (Y fare class up'd to A) or A,D or R and it will award you bonus miles and two segment points.