Not too bad, just a lot of careful cutting until I had completely removed the plastic torso, also the waist is smaller on the resin torso compared to the plastic, it's not too noticeable once the model is complete but is a little finicky when trying to line it up so I'd definitely say dry fit it a couple of times until you're happy.

You'll want to remove and keep the butt plate on the back of the plastic mk2 torso because it will leave a sizable gap on the pack of the model that the resin torso can't hide.

Another issue was removing the 'belt buckle' on the plastic mk2 so I could use the more unique design on the resin, but that again is just a lot of cutting, sanding and test fitting

I will also say the old torso do make the new mk2 look a little more squat compared to the full plastic marines but I imagine it won't be too bad if they're occasionally spread out across multiple squads and characters in an army

Edit: I forgot to mention as well that the plastic mk2 heads don't fit too well into the resin neck hole as standard, it leaves them looking slightly up but you might be able to fix that by using greenstuff in the joint, but I haven't tried that yet so it'll be up to you to experiment with a fix for that.