I have a 2007 Ford Fusion with 6 CD Changer factory stereo + Sirius. Is there a way to remove Sirius and put an HD Radio module in instead?

I'm assuming the Sirius is a plug in unit to the factory stereo. If so, is there a plug in HD radio unit I can swap into it?

Not with the existing audio deck, no.

Yes, you can simply search for "Ford Fusion after market radios." Keep in mind, it is not simply a hot, ground and antenna arrangement anymore. There are buttons on the steering wheel with either FOUR or EIGHT controls for the radio. You want to get an after market radio system that hooks into those too. Fortunately the connections are standardized.