Samo,
On that head, you might try removing it and putting it in the oven for an hour or so at 400 - 500 degrees. (The wife will love it!) Be sure to clean as much oil off of it first as it will smoke A LOT. Put it in with the oven cool and allow it warm up as the oven does. "Bake" it good. Remove it from the oven and quickly bolt it back onto the engine so its well secured as you're reefing on it. Put a large wrench on the COV and hopefully it will give.
If it still won't budge, then repeat the process but this time try using some dry ice to quickly cool ONLY the change over valve while the head is still really hot. Keep the dry ice away from the iron because you don't want to crack it. The ice will shrink the valve and that might be enough to get it to let go.
That's what I'd do.
Good luck!
Steve