Merging Team Project Collections

There are a couple of things that might be causing this.

Make sure you have the latest version of both branches in your local workspace. If you've got an old version of the child branch then TFS will not recognise the changes on the server. This is because all changes (including merges) are done on your local machine before being committed to the server.

If you've attempted a merge, unchecked files from the pending changes, and then checked in the merge. You've just told TFS to ignore those changes (seems stupid I know). If this is the case then you need to use the "tf merge /force" command from the command prompt "tf merge /?" will show you the full syntax of the merge command

Yes, you can.

Upgrading one Collection at a time is not recommended for the following reasons:

• it is the less trodden path, so you may encounter unique problems;
• detaching a TFS collection is a risky operation (I had to escalate to MS Support in some cases especially for TFS 2012).

Another option for you, requiring more time and resources but safer, is to use a staging TFS instance used for migration purpose. The idea is to replicate the existing 2012 (I do not use clone because it is dangerous), installing the same version of SQL and TFS, restore your backups, configure TFS 2012 (this is the most dangerous step, use different accounts etc.), finally upgrade to the same TFS 2017 version you will use in production. At this point, you can detach a collection from staging and be sure it will attach to production without issues.