lifting old patch
I've used a heat gun or hair dryer to remove a patch and other glued material safely. What has worked for me is to pass the gun back and forth over the top of the patch, near an edge, about 6 inches from the surface. In my other hand I have a dull edge butter knife which is used to attempt to lift/seperate the patch at the hot edge. When it lifts easily, without pulling off the layer of hypalon, (usually after 15 seconds or so) I grasp it with pliers and exert a steady and even pull while continuing to pass the heat gun back and forth until the desired piece is removed. If the area being lifted is so wide that the pull is uneven (too spread out), then I shift the lifting point to a smaller area - working in sections that will ensure an even pull on the fabric.
If you are cautious, not trying to lift until the glue is soft, the patch should lift without problems. You will know within the first few seconds of lifting the patch just how much pull is required. If you should begin to pull off bits of the underlying fabric, you are exerting too much pull or the area is not hot enough. If bits of the patch are left on the fabric that's not a problem, they can be sanded smooth. Watch closely for any discoloration, which means the fabric is too hot.
Considertion must be given to having extreme heat over the same spot repeatedly, because of risk to the internal (unseen) layer of material that is bonded to the fabric. Similarly, if your patch is over a seem, you need to be particularly careful so the seam glue does not loosen.