Thanks. I ended up moving the pre-filter, biomax and 2 of the old sponges from the Fluval to the XP3.
I wanted to wait on the water change, but with a fully stocked tank, there is a lot of waste and I didn't want to chance causing any other problems. So I did a 50% pwc. I'm hoping between the old media, the tank walls, DW, rocks and plants, there will be enough bacteria to maintain the ammonia.
I'll check it daily over the next couple of days to make sure everything looks good and let you know what I find, as I'm sure others will run into this too.
As a side note, I have to say, I'm extremely impressed and happy with the XP3! I originally set it up with the power nozzle it comes with, but when I turned it on it was blowing plants and fish all over the place! I ended up turning it off quickly and went with the spray bar. Even with the spray bar, the pressure/current is literally 5x more than with the Fluval 304. I had to aim the spray bar at 45 degree or so towards the surface to help minimize the flow in the tank. Best purchase I've made yet...