Converting a convertible to an express?

It depends. On Most Flybridge SF the Flybridge was a seperate piece that was screwed and glued to the house and it wouldn't take a whole lot to fill in the holes. NOW, if the House looks good without the bridge on it, you've got a winner. If not, you've got lots of modifying to do. But generally a lot cheaper to buy an existing boat and better resale unless you get your conversion really right.

Additionally some SF models use some of the flybridge structure for passageways for air condition ducting.

I just saw an older 33' Egg Flybridge cut up in a 20 yard dumpster!

That could have been a cool little project.

Its a good thing I didn't know it was getting cut up, because my wife would have killed me if I took on that as a project.

At one time large expresses boats were the rage on the sportfish scene.
and it was becoming common to see some convertibles converted into express boats.

Most recently the sportfish scene in the Northeast has shifted from large Convertibles, to go fast multiple outboard center consoles, and the latest craze offshore is the Downeaster.