I have mixed feelings. I started with Milsurps about 20 years ago. Enfields, Swedes, Argentines, Brazilians, Steyr Mannlichers, FR 8's, 03A3, some others here and there, and this year I acquired a few K-31's. I have some really pristine examples, most are excellent, and all but one with matching numbers. In the most "general" average sense, I would estimate that my 20 year old milsurps have increased in value about 3 to 4 times what I paid for them. Of course the more prime examples of the more desireable models like the Swedes lead the pack, and some others don't appreciate that much. I figure that equates to between 6% and 7% annual appreciation over that time. That's not like a hot stock, but it's better than many other investments. I would have a hard time even thinking of chopping up an almost perfect Swede or Argentine. Still, I feel like I bought them for me to shoot and enjoy, and if I want to modify one or refinish one, that's what I bought it for - for me to enjoy. I don't own any high dollar .30 carbines, or anything THAT costly, and I would not touch a truly pristine milsurp - no reason to.
I did have one Argentine (which I paid about $100 for) made into a beautiful '06 Mannlicher style carbine, but most of my other modifications are reversible - scout mounts, mojo sights, etc. I keep all the original pieces to return to the rifle if necessary. I do intend to do some stock refinishing on my K-31's - because I enjoy them more when the wood looks good.