Sorry, cannot re-melt them.
The easy answer is that you can cut them up into small pieces and use them as fillers on your new molds. In other words, if your mold wall is a half inch thick, you can pour a little less and put 1/4" pieces between the item being molded and the wall to take up space. The old material will adhere perfectly to the first if it is clean.
The not so easy answer is to ....... I wonder if I should even bring this up..
...... to modify the mold. For example, let us say that we have an old minnow bait and the mold is just worn out. You can trim out the damaged pieces, down to clean material, then put new material in and place your master back in and press it together and the new will bond with the old and now you have a new mold. Works great for me. Or, perhaps you have a minnow bait that you want to make thicker or longer. You could again trim out the guts, making the cavity large enough for the new master, place new material in and the new master, filling the cavity, leaving you with a "new mold". Again, this is the not so easy answer and it does take a bit of planning, a little natural skill, and practice...... but it does work.
If the above two options are not good for you, then all I know if is to trash them.
Hope this helps some.
Steve