In my company (www.negotium.com ) we are trying to achieve a significant migration for us people of development: TFS 2010 to visual studio online (ex tfs online).
This will bring many new opportunities and tools to our way of developing and it will allow our IT Department to decommission a (several in fact) machine. I'm preparing a more complete article on the benefits of the solution online from the online edition.
Between TFS 2010 and VS Online we had:
We are talking about a migration of 3 versions (it depends on how we count there). I would like to say that we use this free migration tool http://www.opshub.com/main/index.php/products/vsomigrationutility
Other information to be taken into account, we are interested by the migration of the work items (tasks, bugs...), security and source control (including notes, links etc). Build plans will have to be redone anyway and deployments/test agents will be managed by VSO / Azure.
Differences between models
We however had difficulties during our migration tests: differences between models of organizational processes (the thing you choose at the beginning of the creation of a project) between the version on prem and online.
The solution is pretty simple, but it took us time to find it so I post here in case that makes you save some. I would like to thank Jérémie who is working with me on this migration project.
We only had to rename existing fields so that they match the naming convention used on VSO before migrating. Info: TFS power tools have a feature for it but because it was system fields, we had to do it in command line. (you must of course be the tfs server admin)
witadmin changefield / collection: http://negotiumtfs:8080/tfs/DefaultCollection / n: System.HyperLinkCount/name: "Hyperlink Count"
more information here
Once this done on faulting fields we could go on our tests.
I will keep you informed if we ever meet other difficulties.