Hi Fred, firstly you would have to break the existing joint without damaging the tops which may not be straight forward. If they have been glued or have a biscuit joint you will almost definitely do damage. Secondly, remachining the existing joints would be difficult as well. If the new worktop jig is different from the original they may have different template shapes and even if the jig used is the original one, lining up to the existing cuts will be very difficult. You would have to make minute adjustments to get what would be just a cleaning up of the original cut. I have personally remachined an existing joint but it was very fiddly and it was a relatively new joint I had done with the same jig.