We are attempting to migrate from one Visual Studio Online
account to another Visual Studio Online
account. This is different from an on-premise
You can consider using TFS Integration Platform
For others that stumble across this thread, I have found this - the Visual Studio Team Services Sync & Migration Engine an open source project that appears to do TFS-TFS, TFS-VSTS, VSTS-TFS and VSTS-VSTS migration. It is an open source project initiated with the same purpose as the TFS Integration Platform, but to address some of the complexities (and the fact that it is now deprecated).
From the web site:
Visual Studio Team Services Sync & Migration Engine allows you to bulk edit data in Microsoft Team Foundation Server (TFS) and Visual Studio Team Services (VSTS). Supports both migration and bulk update scenarios.