Boonex have promised that there will be a final 6.1.5 version for bug fixes after 7 stable is out. so you can keep your version of 6.1 and you'll loose nothing.

as to upgrading to 7 from 6, this will really only happen for sites that have no mods. if you have modified your site in anyway, it will not upgrade automatically, you'll have to do a manual upgrade,

manual upgrade means:
install dolphin 7 fresh and them move your sites media files over to 7 from 6 and the tables in the relevent database see more tables ensureing that the tables fit with 7's tables.
no template will upgrade without you working on it.
anyone that just changed the uni css files to change the sites looks will loose them too.