I want to primarily use Seachem's Flourite as the
substrate with some regular gravel on top. My current substrate is a
very thin layer of regular gravel.

Any advice on how to make the switch

You can siphon out the existing gravel, keep it in the tank water. Add
fluorite, it is very messy, you can rinse forever and still makes a cloudy
mess. The mess will clear with the filter and then you can reintorduce the
gravel. If the gravel doesn't look good you can lay down a piece of open mesh
fabric (tulle) and pull it out after a week. The bacteria should have migrated
by that time.