zbrush question

I'm a total noob with Zbrush, so it's prolly right in front of my nose but can't seem to find the answer (also not via Google).

I saved my document yesterday, and now when I open it the edit button is suddenly greyed out. I read something about reïmporting your mesh but when I do that I (obviously) get the unedited version.

Help would be real welcome.

e: Mmh, read somewhere that you should save as a Tool instead of a Document. Any way to still transform my document into a tool?