I have uploaded a tool i’ve been working on for a year.
WI Migrator performs a migration of TFS work items environment from a Team Project to another, cross TFS collections / servers.
Great solution for migrating between two live TFS environments, including migration to the cloud, from the cloud or between clouds.
WI Migrator duplicates the work items, restores the links between them,
and recreates the test plans and the test suites. Work items attachments included.
TFS Servers supported: TFS 2010/2012/2013/2015/ Visual Studio Online.
You can give it a try.
a trial version is on vs gallery. with the trial you can migrate up to 50 work items. The tool can be purchased and the price depends on work items count.
WI Migrator for Visual Studio 2015
WI Migrator for Visual Studio 2013