Has anyone ever done that?
I got curious about the Sony hybrid tips that keep getting mentioned around here, but didn't feel like ordering a set. I figured, since I have two pair of Comply tips that came with my UE 700, might as well DIY and do some surgery. It was surprisingly easy to get the plastic sleeves out of the Comply tips. I took a hobby knife and just slowly cut around the spot in between the sleeve and the foam, and eventually the sleeve came out with little resistance. I then inverted my silicone tips and slipped the now foam-only Comply tips onto the stems. I found that it's easier to slip them on backwards, since the hole where the plastic sleeve used to be is bigger. I flipped the silicone back and made my very own hybrids.
I'll update with pics and reports on isolation and comfort later. In the meantime, I invite any other (mildly) adventurous Head-Fiers to do the same and report their findings.