Pro-sites: Use of particular plugin on upgrade

Hi.
I would like to use a very basic site for initial sign-ups, that does not use one particular plugin.
I would then like to make this plugin available as an upgrade.
Could you please give me a quick explanation as to how to achieve this? I am fresh to Pro-sites!
I could experiment, but do not want to mess up a definitive site critically!!!!!!

Thanking you in advance.

Regards
John