The only thought I have is getting a small corner or side up a bit and then using a nice flush cut saw so the teeth dont marr the wood or seat and is only getting at the glue.  A heat gun would probably kill the glue but Im afraid of what it would do to your seat (and boat epoxy.) Not sure if that helps or not. Good luck!

I've done this a couple of times. Since I used rubber cement to glue them down, they came up pretty easily. Try using a broad putty knife. At worst, you will cut away all but the outer layer of foam, which you can then sand down enough to re-glue the seat. On my Merganser, which is nearly identical to your Shearwater, I use the hip pads to hold the seat firmly in place without glue, but I can still adjust it with some effort. -Wes

I am soon going to be putting in the happy bottom seat.  I see here that you used rubber cement to glue down the seat.  Did you varnish there or not?  I have left mine sanded, epoxy only, in that area.  My boat is a Matunuck (see my post, please, it looks good, if I do say so myself) The seat comes with hip pads.  But I can add minicell foam around it, and of course in back for a backrest.  Next weekend I will start the carving of the foam.  My partner Dave suggest that we install the seats using Velcro until we get to a happy position for our happy bottom seats. 

We will be taking our matunucks to the Sacramento exhibit for CLC.  Anybody out there local who wishes to see our finished products should come take a look, as Joey Schott invited us to bring them down.