I've had some success using a hair dryer to free up Agfa grease. I typically take the shutter off the camera first to avoid damaging bellows or struts. I heat the shutter in my hand until it is almost too hot to hold and then press and turn the front element against a rubber mat. My wife's silicone cooking covers work great. Once you get it to move, turn it to infinity so you can index the lens. Please be aware that the threads of the middle element may come loose instead of the front element, if that happens, you'll probably have to collimate the lens once you get the two elements apart. Also, once the lens cools, the grease may re-harden so you can't turn it again.
Take off the front trim ring (while set to infinity) by loosening the small set screws, don't remove them and don't let the lens turn. Using an exacto knife, make a small mark on the rim of the front element across from the focus mark on the shutter. Then turn the lens in until it bottoms out in the mount. Make a second mark on the lens mount right across from the mark you just made on the lens rim. This mark will help you make sure you have started the helical threads in the right place. Turn the front lens all the way out and clean the grease off the threads. I have been using 90% Isopropyl alcohol as a solvent. I have a Prontor service manual that say to use gasoline! I then use sewing machine oil, just two drops, on the threads. It soaks into the grease remnants and makes for a smooth movement.
An alternative to using heat is to soak the front element in Isopropyl alcohol to loosen the threads. I usually do this if I need to clean the shutter since that's what I use to free up sticky shutters.
When you replace the front element, the helical threads can be started in several different positions. The correct one will have the marks on the lens mount and lens rim line up when the lens is turned all the way in. Then turn the lens back out until the mark lines up with the shutter focus mark. Replace the trim ring with the infinity mark lining up with the shutter mark.