Pop the bolts out on the pylon posts, get ya a big heavy rubber mallet and WD40. let the lube soak in it for about 20 mins then whack the tramp frame upwards to release the hull a little at a time on each pylon. Once you get it started the hull will come off easily. There's a really good chance the casting holes wont line up with the new pylon holes/hull so have a 1/2" drill bit and a drill handy. Prob a good idea to take one of the bolts you removed to the hardware store and get new stainless bolts and nylock nuts.
And before you do that, drop the mast, remove all the standing rigging from the old hull (best to remove the mast/rigging entirely so you can man-handle the frame), remove the rudder system, and loosen the trampoline. After you've done that, grab one bow and shake it up and down vigorously. The racking will help break the corrosion on the pylon/casting.
If you're not concerned about damaging the old hull, stick a 4' long 2x4 between the hull and casting (NOT the sidebar) and use it to lever off the casting from the pylon (orient the 2x4 with the wide side vertical). Dead-blow hammer works better than a rubber mallet. Bigger is better.