Cutout Diameter

Here's how I did the same thing with my HT mains.  I decided to switch out the 4.5" mids to another line and the new ones didn't quite fit into the routed holes that I made.

You can see that the holes were mortised so that I could flush-mount the driver.  That left the idea of using a jigsaw out of the question because I needed a perfect circle with the proper depth for the new mids.

I use a jig called a Jasper jig, which needs to pivot on the center of the cutout.  I needed to fill the center of these holes up with solid MDF and drill a center hole so that I could rout a new cutout.

1.   Cut a disk that will fit into the hole you now have.

2.  Cut a backing brace that is narrow enough to fit through the existing hole, and long enough to overlap the sides of the hole.

3.  Mark and predrill starter holes in the backing brace to line up with two or more of the speaker mounting holes.  Then secure the backing brace to the inside of the box, extending across the cutout.

4.  Place the disk into the hole and predrill starter holes that will secure it to the brace.  Secure them together with wood screws.

5.  Through trial (but no error), find the center of the disk by using the router and jig.  Secure the jig to the center of the disk and re-rout the hole to the proper size.

Oh...I wrote all this and then I re-read your post.  Oh well...

My home HT main speakers, designed to flank the TV.  I changed the mids to the high Vifa line, so they look better than the ones shown in these (link below).  These were a buyout deal and sounded pretty good but I didn't like the metallic color and the fact that they were made to have an overlapping lip over the cutout.  The new ones match the rest of the speakers better and sound better, but I haven't taken any pictures since I made the switch.

I showcased them when I got the project done HERE.