Mattress Surgery Questions

I ordered a simple pocket coil mattress with 2" talalay foam on both sides and an organic cotton cover (sewn, non-zippable).
The ILD I asked for was about a “medium” by Latex International’s standards (mid-20s).
Without telling me, the seller ordered a 36ILD instead (x-firm) because he didn’t trust that I knew what I would need
I am getting a lot of latex pushback and am in pain while sleeping (WHEN I’m able to sleep.)

I’m not doing any more exchanges with anyone. The coils are good and I have a nice 2" piece of medium dunlop from Arizona here at the house, about a mid-20s ILD. I would like to cut the mattress, take out the piece he put in and insert my own.

Note: This is a 2-sided mattress so the bottom also has 2" of the x-firm talalay; I will leave that for support in case the coil unit needs it for support.

My question is, can this be performed on a bed that is not a pillowtop? I am not a seamstress and concerned about the sewing part of it, especially since the cover is tight. Is there a certain sharp knife, razor blade etc that people find best to undo stitching - and is there a certain part of or place on the mattress that’s best to cut along?

I am also perusing this site for links about this.