Ditto for knowing whether the liner is a structural part of the boat. Seems likely.
Obviously, it's your's to do with as you please, but because you asked, I think you're going to find other much better places to spend your time and money on an old boat. I KNOW you will.
A crash compartment? Seriously? Do you hit a lot of stuff (tongue in cheek)? How many 25' sailboats sink from head on collisions vs. other cause? I'll betcha darn few. Try fire, through-hulls, and general stupidity. Also, bear in mind not many of them sink.
I understand going for better ergos down below, but it's still going to be a 25' boat when you're done, so you're not going to gain much. What are you REALLY planning to do down there with the ergos that you hope will be better? Just sail the boat. Keep it seaworthy and shipshape. That's no small undertaking on an older boat. You'll use the cabin to overnight sometimes, get out of the weather, cook chow, store your gear, and maybe get laid. Rearranging will not likely enhance your experience.
I used to sail a Grampian 26, and I really loved that boat. I did a lot of repair and upgrades to it outside and inside. The bulkheads in that boat were covered with formica, and I thought it looked really cheesy. I paneled over it and added trim here and there on the interior. I wanted a bigger head on the boat with a shower and figured out I could put a shower in the forepeak in place of the v bunks. Glad I never did that. Looking back, I'm glad I did all the work on the outside, but the stuff I did non the inside was just a waste of time, effort and money. Fix the broken stuff, rewire where needed, and just enjoy it.