Does your client want to break out from the existing older version of TFS (2015 and later) to Azure DevOps Services? Are you looking to upgrade only one project within the TFS collection to Azure DevOps Services? 
Keep reading to find out how it is possible to migrate a single project without having to do yet another upgrade of TFS or migrate the entire collection to Azure DevOps Services.
One of the top reasons that I hear from customers is that they do not want to go through yet another TFS upgrade and so move to Azure DevOps. With Azure DevOps, you don’t have to worry about downtime, maintenance, upgrades, patches, or infrastructure. Microsoft does all of that for you. You can start focusing on planning and delivering constant value to your business while Microsoft works in the backend on reliability and security.
Working on TFS since its inception, vast experience with core concepts of TFS and having implemented Azure DevOps for 1000+ customers, we have had some unique feedback from our customers and one of those was to create a tool – VSTS Copy Project which is already a favorite extension in the Azure DevOps marketplace where we have had customers copy their existing Azure DevOps projects from source organization to destination organization within the same tenant, across different tenants. 
Recently, we have released another flavor of the same tool which can migrate projects from TFS to Azure DevOps Services. This tool is developed in DOTNET language and uses REST APIs, SDKs exposed by Microsoft and will migrate a single project from TFS 2015 and later versions to Azure DevOps Services without having to migrate the complete collection or even better not bring the whole collection (in which the project resides) offline for the time of upgrade. Isn’t this an awesome feature!
The tool seamlessly migrates work items with history, links, attachments, revisions, commit IDs, to name a few. We have developed a PowerShell wrapper for GIT-TFS migration which can migrate multiple repos at once and validates the GIT repositories in Azure DevOps post migration. For the Process customization done in TFS, our DevOps experts will do thorough analysis and will come up customization approaches and implement in the target Azure DevOps Service. The classic vNext builds and release definitions will be migrated as is to the destination and the test plans, suites are migrated and created retaining their original hierarchy. Although there are few limitations which are purely due to the unavailability of features in the APIs, three-fourth of the TFS project data can be migrated to Azure DevOps effortlessly.