Since we have the tools and technology to make certain that the existing inletting in the BTFE would not leave unsightly gaps if converted to a splinter, I absolutely would not just start cutting, and hope for the best.
I also would not even consider splicing the edges of the existing forend wood to cover possible gaps, because there is going to be a glue joint line that would not be able to be perfectly hidden full -length in the checkering. My personal choice would be to start looking for a new piece of wood with closely matching grain and pore size, and start whittling a new splinter forend. That way, you would still retain the matching BTFE, should you sell the gun. It really isn't that hard to inlet and fit a shotgun forend. I say that knowing you have the tools and the ability. Checkering will need to be done by you or someone either way. And if you screw up, you won't be out as much as you would if remodeling the BTFE did not go well. Besides, your biggest challenge is not going to be matching the wood itself. The real challenge will be matching the stain and finish of the buttstock, assuming you'd rather not refinish everything. And that will be a problem to solve whether you use the existing forend wood, or start fresh with another blank.
Anyway, that's my advice. Tale it or leave it while you can, because I am seriously questioning the wisdom of giving free advice on this Double-Standards forum while selective moderation is going on, and certain sensitive "special" people are still permitted to submit and cheer-on personal insults. I can leave the free advice giving to someone who doesn't even know the difference between feather-crotch black walnut and thin shell walnut.
