One or more domains + Upgrading S2member Pro to Multiple

Hi, I sent a message to WPsharks a few days ago but no reply. Perhaps someone here knows the answer?

I’m setting up my site on a staging domain eg
If I pay for S2 Member Pro and then move my developed site to the top level, ie does this count as more than one domain?

Also, if I later decide to set up more sites, can I upgrade to multiple site version for the difference in price ie $100, or will I have to pay in full over again, as if I’d never had the Pro version?