When you cycle a tank, you are building a bacteria base in the rock and sand in your tank. If you are following the 1 lbs per gallon of rock and sand, then changing out filter media doesn't matter at all because the bacteria are in the rock and sand...they are your main filtration in a SW
In terms of switching to a sump, just some advice as I originally was scared of making my own sump when I first started and picked up one of those guys. It was a nice unit that was well built...but there is no room in it. You can save yourself a TON of money by just picking up some silicone, a cheap tank from your LFS
, and any Lowes or Home Depot will cut you glass to scale. It took me about 20 minutes to put it together how I wanted it to be, and there are tons of ideas and guides on how to do this out there to help if you get confused. It was a much better idea to spend a total of 30 bucks to build a sump than over a hundred for one I wasn't happy with at all.