At the very least, you'll probably get a funny look from the agent. At the worst (although not the very worst), they may charge you $100. In my experience, even if the ORD-MIA is available, they will rebook you direct to TPA.

Here's something to try, I've never even thought about doing it until just now. See if you can change your flight at the electronic rebooking centers in ORD. Even if you can only select a later morning flight, at least it's closer to your projected departure, and therefore less likely to raise eyebrows. I'm sure somebody will post that this won't work. Just a thought.