Lots of ways to alter a mold a little. The putties mentioned already are one way. I'm thinking that a hammer and chisel is maybe over-kill. The Dremel use has to be done carefully since a slip cannot be un-done very easy and can really mess up a mold. A drill press has some use in mold conversion at times. Probably the best way to change a cavity [cavities] as far as hooks goes is to simply lay a hook in the cavity you want to alter, carefully close the mold holding enough pressure on the handles to prevent the hook from slipping and then setting the closed mold, still holing the handles tight together, flat on a vise and then give the mold a swat or two using a plastic or rubber faced hammer. The aluminum casting will imprint the hook perfectly. Check to be sure the mold closes completely with all of the hooks in place in all of the cavities after you have done all of your alterations. Some aluminum burr may get pushed up where the hooks were pressed into the casting and not allow complete closure. In that case take a medium flat file and run it lightly across the flat surfaces on the mold checking after each cut is made on both sides. When the mold closes flat with the hooks you want, you're done.
Any time you are going to attempt changing a mold, spend plenty of un-distracted time laying hooks in place and visualizing what needs to be done before you set out on this trip. Then when you start the project make sure you can work un-interrupted so you can focus on what you're doing. Its really hard to ruin a mold but it doesn't take much to make it less than what it was new. If you are not handy with tools, get someone to help you who is.
