Rick,
I never ever really tried it on a production server but on my test-servers I can switch easy from server only mode to server & gateway mode easy by the admin panel.
Login as admin (instead of root) and choose 'configure' and you can re-configure your server easy and if I'm right you won't loose anything, even going from S&G to SO and back to S&G seems to keep the data. The keys simply get disabled and enabled as change, the rest sticks.
Harro
PS : a full back-up is never useless.... !!!!