Not really a jig or a fixture but, a jig-like method to solve a problem. We've probably all seen this somewhere and some folks even sell router bit "sets" to do this.
Anyway, I had a profile inside a frame that is to accept a water container of a certain shape. I goofed and the cutout needed to be a bit (no pun intended) bigger. Here's the 'before':
Setup the bit of your choice to take an 1/8" bite and set the height to use the existing edge path as a guide. Make your first pass.
Change to a flush bit (or change bearings), flip the piece and use the "new" edge as a reference for the flush cut. Pardon the burn marks, they get taken care of by the flush pass.
I did it twice as I needed to increase the dimension by 1/4" and didn't want to splinter the ash with a big bite. Here's the 'after':
HTH someone else out there.