Good morning, Kevin!
The jelly will liquefy if you warm it up for too long or if your slices are too thin. But if your headcheese is pretty dense, you can slice about 1/2" thick or more, then quickly sear both sides in a frying pan for about 30 seconds each side to take the chill off. If you heat it for too long the whole slice will break up, but you can get the outsides a little warm and melty while the inside stays firm. When I make trotter-hock terrine my husband and I often eat it for breakfast this way topped with a fried egg…we use the same pan we cook the eggs in, just add in the slices after we turn the eggs. I've even seen some recipes that use a breading on the outside and sear both sides to form a crust, but I haven't tried this. I think you need to have pretty thick slices for this to work, like 1" thick.
Have a great Sunday!