I recently uploaded a custom map-variant on my Halo 3 file share, but I have edited the map in forge since then. Will the updated version be on my file share or will I have to re-upload it?
No it wouldn't, what if you where forging on the map, re-did a part, and saved it to come back in the morning and finish, if it updated at night, people would download an un-finished map... it is not that hard to do manually.