I shared information in an ancestry.com family tree with a relative who wanted to make additions to her side of the family. She was adamant about not wanting to create an account, so I downloaded the tree in GEDCOM format. I now have a large amount of additions in GEDCOM format that I would like to fold into my existing tree. Unfortunately, I couldn't figure out how to do that. Is it even possible, or do I have to go through all the changes manually?
Currently, the only way to sync a tree with Ancestry is with Family Tree Maker. The functionality of the current syncing process known as TreeSync is said to be hit and miss. Some people say that it works very well. Others have nothing but problems with it.
If you don't already have a version of Family Tree Maker, then you can purchase it from Software MacKiev. MacKiev recently acquired the rights to Family Tree Maker from Ancestry. You can try out the current version of TreeSync and see if it works for you.
On November 5, Jack Minsky of MacKiev wrote (as reported by Randy Seaver), that they are developing new syncing technology to replace the troublesome TreeSync that exists today. To keep up with the latest news about the new sync technology, you can join the FTM mailing list.
There is an alternative. Ancestry has also allowed RootsMagic to develop syncing with its Ancestry.com site. RootsMagic announced that it would be working to develop the syncing in February 2016, but they have not yet completed that feature yet.
So your options as of November 2016 are: (1) you can try the current version of Family Tree Maker, (2) you can wait for the next version of Family Tree Maker or RootsMagic, or (3) you can manually enter all your changes yourself onto Ancestry.com.