Don't re-compile the old ones, do not use the editor directly... It works somewhat better to transfer the parameters directly using a text program...
How To Do This Sorcery:
1) Start up the mission editor and save a new blank mission on the terrain you desire...
2) Find the old mission you want to convert and open it in a text editor... At this point you will see a barrage of simple parameters... Copy ALL of this to your clipboard...
3) Open the new mission file you just made from STEP 1, paste, and save...
4) Attempt to open your converted file in the mission editor, make any changes, and re-compile... Once done, attempt to upload it and test in a custom arena....