do not understand how the top cap and hull layer together
The deck and hulls are built separately and then glued together in a jig (this can be seen very clearly on "red glue" boats since the red glue is visible at the hull/deck seam).
After further thought it would seem that removing the top cap would allow the hull to move out of shape, especially once the inner glass and foam have been removed.
I think this would be a MAJOR issue. You'd more than likely have to build a jig that you can lock the hull into to hold it's shape before you start tearing things apart. The foam sandwich gives the sides of the hulls all of their rigidity, once you remove the inner layer of glass, you'll be left with only a flimsy, thin, outer layer.
So the question is, why do you want to go away from the foam core/sandwich construction? A solid fiberglass hull will probably have to be close to the same thickness as the current sandwich in order to be stiff enough. This would equate to a serious weight increase. I think you'd be better off replacing the foam core using sheets of divinycell and then laminating a new inner layer of glass. Probably first cut the core to the correct size, apply resin, then pull the core against the outer skin using screws through the hull skin, distribute the load using long strips of wood. You might also consider installing bulkheads for increased stiffness.