I\'m migrating our local TFS to VSTS (Cloud TFS). Everything worked fine so far, but now I want to migrate all the existing workitems to the VSTS. I have searched the inter
TFS has three options for scoping and scaling data - deployments, team project collections, and team projects. Team projects are used to encapsulate the assets of individual software projects, including source code, work items, and so on.
If you don't mind losing work item history, you could just pull over work items, using Excel. Details steps please refer this blog- -Migrating TFS Work Items to a new Team Project using Excel (TFS/VSO/VSTS) or this thread- -Migrating Work Items from TFS 2017 to VSTS
If you have no work item customisation, then OpsHub is also an option.
If you have work item customisation and need keeping histroy , then you are going to have to use TFS Integration Platform.
You could also use some customize powershell script or 3-party tool such as VSTS Sync Migration Tools for migrating WI with attachments.
More ways and detail explanation please have a look at jessehouwing's answer in this question: How to migrate work items from TFS to VS Team Services (VS Online)