What hosta did you buy and do you a couple of photos? There is an excellent post very recently about separating the green from her Brother Stephan. Check that out if you can and wait for others to respond.

Newhostalady has the post about Brother Stephan at the top of page two, and I myself just posted how I separated out my all blue leaves from my June. I am no expert, but for whatever reason, hostas will do this, revert to the original plant that was used to make them. From what I recently learned, when a hosta begins to revert, you can prevent that by separating out the all green leaves. The green ones are more aggressive in their growth, for some reason that escapes me now and the sooner you separate them the more likely you are to retain the original plant. Again, adding a photo will help with comments.