Open the client file that contains the tab you wish to move. There is a new toolbar button available:

Move tab options are displayed:

Pick the client file you wish to move the tab to using the search button:

Choose if you want the tab to appear at the front or back of the file.

Click OK to move the selected tab.
The tab has been moved to the new client file:

Please note if you wish to give users this feature you will need to amend their permissions in User Maintenance.