changing domain and SEO

Question... I would like to change my Cname on photoshelter (www.RobGreebonPhotography.com). This change would not affect my photoshelter domain name (www.RobGreebon.Photoshelter.com). This particular Cname would cease to point to the photoshelter site.

My question is... how would this affect various galleries and SEO? Right now I rank on page one for several keywords in several galleries and I do not want to affect those rankings. Would changing the Cname trickle down to affect the rankings of galleries?

Does this make sense?

Thanks,

Rob