Very cool, I can visualize the party with the Advents! I love how they play loud. I think you would be creating more work for yourself if you bust the crossover plate out. Reapplying the plate may not allow a good acoustic suspension seal. You'd have to work from within the woofer hole anyway. I see two options, remove the crossover components and update the components. If you do this, you could mount everything on another board that would attach inside, leaving wires to the switch long enough to allow mounting the switch in its hole.
The second option is to work through the woofer hole. maybe one of the kids would be interested in helping to restore these.
I'm more restore oriented, leaning toward using original fried egg tweeters. How about some pictures of the workhorses?
mark