If both sites share a common root domain, you can share cookies by using that root domain as the cookie domain for both sites. This way, a login stored in the cookie for one site will also apply to the other site. Session cookies will be shared as well.
To force Craft to use only the root domain for it's cookies, use the defaultCookieDomain directive.
If you are worried about data loss, I would duplicate the site locally as a test install and try on there. If that works fine, I would backup files and DB again before modifying groups on my dev install.
TBH I don't recall having data losses by switching goups, but that's not something I typically do every day.