Wow.. big job.
24 foot swing keel? What's on the interior surfaces now? Anything?
If the deck surface is in good shape it would be nice to do all this from the inside... but working with coring and fabric upside down is a pretty daunting thought... is this boat small enough, and do you have the setup to be able to roll her over? and then would there be access to work inside? Gravity would be your friend then (at least until you rolled her back..
) You'd also need to worry about supporting the deck to maintain it's original shape.
Removing the outer skin and redoing the deck afterwards is more work still, though you've the advantage of working with glass in a more ventilated environment and you can do much of the work from alongside the hull. However it's very difficult to get a professional looking finish on the deck, working from below you can always cover it up with something.
Either way you look at it this is a lousy, big job. Doing it in stages may be easiest, but doing it in such a way that your final skins end up being continuous membranes instead of sections would give the best result.
Is the delam wet? if so you're looking at some dryout time once you've opened it up, and again doing it all at once would probably be best.
I don't envy you.. hope you didn't pay much for this project!