Removing / changing substrate?

Anyone have any experience with changing their substrate?

I have a fully cycled and fully stocked 1 year old 46g tank, with
plants. Originally when I set it up, the substrate was a mix of 1/2
fluorite, 1/2 resin-coated aquarium gravel. It was stupid for me to
mix in the aqua-gravel, but I didn't know better at the time.

I want to start replacing the original substrate- basically to get rid
of the fake-gravel. Anyone ever do this? Any tips?

What I plan to do is remove 4 cups of substrate and replace it with 4
cups of new fluorite that has been cleaned and soaked with tank water.
Is it Ok to do this on a regular, but methodical, basis? My plan
would be to do this twice per week. My goal is to remove most of the
fake-gravel, and if it takes months, so be it. I figure 4 cups at a
time won't screw too much with the bio-filtration living in my
substrate?

Will this idea work? It's a 46g tank, and I do change out 10 gallons
every week. PH is a steady 8, good KH, no ammonia or nitrites;
nitrates are less than 20, and I am running a well-established Fluval
304.

Am I taking any risks by attempting this?