synchronizing family trees

I have my tree on Family Tree Maker on my home computer. Also, I have uploaded a tree to Ancestry (and to MyHeritage, WeRelate). For the tree that actually resides on Ancestry, people have attached various source documentation to it, e.g., census images, that I find helpful and would like to keep. At the same time, because I have trees up in other places, I would like to keep a master file on my home computer and upload revised gedcoms/trees from time to time. However, now I am worried about losing the helpful stuff that people have attached to my tree on Ancestry if I upload a revised tree. Is there a way, essentially, to download back to my computer a gedcom in reverse -- a tree that contains the updated information, which I can work on and (when I am satisfied that the updated information is in fact correct) then replace my master file with?

I hope this query is coherent!