There might be a lot of tools available to migrate from TFS to Visual Studio Online (VSO) but one of the tool that I found might be useful is OpsHub. I went through some more links provided by Visual Studio and also Visual Studio Gallery to download the OpsHub tool. I tried to prototype a scenario here using trial version of TFS and creating an account in Visual Studio Online to show how the migration happens. I was not successful due to the limitations of TFS trial version but I feel we should give a try using this tool.
First of all I have created a VSO account as below
Once the account is created I thought of creating a Team Project as below
Once the Project is created you can connect to VSO Team Project through TFS using your Microsoft Account.
Once you had verified that you are able to connect to VSO Team Project, Run OpsHub that you have installed to migrate your TFS solution to VSO. There are some limitations which you can learn more in the links provided at the initial stage.
Installation of OpsHub is complete. Now we need to start the migration process.
You might face the above issue while the Migration process starts. In order to resolve this you need to add the account Project Collection Service Accounts so that you have administrative permissions to have the migration process. This can be done in VSO and this link can help you in order to do that.
While I was in the Migration Process I encountered an issue due to some limitations.
You can have a look into the following link to know what can be done in order to resolve this issue. In one forum I found that Older version does not support sync of test suite, run, result entity. Which has been added in 22.214.171.124. This free utility is not supported with TFS2013.3. You can try it with TFS2013 (Not update 3). It should work as expected.
Unfortunately I could had attempted at this point and am not able to progress further. I am sure this utility will be really helpful in the migration process.