Update: it was pretty easy to do, just a bit time consuming.
Use a utility knife around the edges of the arcylic/GRP portlight flange and slowly open it up. We put small wedges to hold a gap between the acrylic and GRP to ease further cutting. The thickness of the GRP means there is very little risk of breaking the lens, IMO.
Being that it's silicone, the sealant released quite easily with a little leverage once you got far enough around.
Once it was off, the real work began. We spent a lot of time cleaning off the old silicone with a wood chisel to get the big pieces off. Then it was fingernails and rubbing with your fingers. A trick I learned was to rub a flour/water paste on the surfaces to highlight any small areas you may have missed. The flour sticks to the silicone and makes it very apparent what you have missed!
Final cleanse with microfiber and rubbing alcohol and then apply a LOT of silicone (most of a 300ml tube) and screw the lens back on, using opposite corners. I asked Olivier if we needed to let the silicone set a bit to avoid it all squeezing out when you tighten the 8 screws, but he said that Amel screws it down immediately to fill all the gaps and we should do the same. :)
I considered using Sika 291i but found that a lot of those adhesive/sealants are not compatible with acrylic. We could have used Sika 295 UV, but the preparation was far more involved and the seal would fail unless it's done perfectly. If silicone was good enough for the Amel Shipyard, it's good enough for me!
Ps - I didn't bother taping the lens or the outside of the portlight opening. Silicone doesn't really stick well to either, so getting off the excess was quite easy.
PPS - When I was asking around for tips on doing it, the other boat owners who looked at the portlight said the same thing most people say when they see my Amel up close: "Wow, they made that really robust!"
Hope this helps someone...
Amel 54 #69 Tengah