I did an event yesterday on using the TFS Database Import Service to do migrations from on premises TFS to VSTS.
During the presentation I discussed some of the other migration options available. Not everyone needs a high fidelity migration, bring everything over. Some teams may want to just bring over their current source or just a subset of their source. Maybe they are making a major change in work practices and want to start anew on VSTS.
To try to give an idea of the options I have produced this flow chart to help with the choices
Click for a PDF version
It mentions a few 3rd party tools in the flowchart, so here are some useful links
Also, if you find yourself in the orange box at the bottom and don’t want to use the TFS Database Import Service for some reason, have a look at this post I did on Microsoft’s UK Developers site. It might give you some ideas