Hard to say without knowing just how the installation
was done. If the silicone was just used as a bedding compound on the interior/exterior, I would use a single-edged razor followed by a STIFF putty knife to break to bond all around, as best you can. The pull or pry the interior/exterior rings off.
If they are plastic, Mark Plastics actually used to use massive amounts of silicon around the edges of the portlights
where they ran through the hull
, so you couldn't cut the silicon, it was necessary to break out the glazing and then "implode" the trim rings to pull the away from the hull
thickness. Followed up by chewing out whatever silicone was left.
There are some release solvents, Home Depot sells a pale (opaque) "natural orange" goo that will loosen silicone but also removes paint
and as a goo it doesn't penetrate well. They also sell a "rocket fuel" stripper in a tall aerosol spray can, and I can tell you that will penetrate and dissolve and release almost anything--but it also finds ay bare skin and burns like all hell, you have to be very
careful using it.
So if a box of single-edged razor blades (or one of those fancy oscillating saws) and a putty knife will work, that may still be the best way to go.
Look into 3M "VHB" tape to seal the new ones in. It isn't your grandpa's old double-stick tape, it is structural quality, but can be released with solvents.