Our company had the same problem 2 months ago and spent more than 2 months scratching my head and wandering what to do. We had project running with outsourcing company and we decided to have the repository with them. When the project was over we wanted to migrate the project to our own VSO.
I was surprised to find out that moving all your changes history is possible only with EXPENSIVE software. No way to pay 5K to move 2-3 projects. We asked Microsoft support and they said we can only change ownership to entire VSO account or pay. Well played MS!
I started googling and asking people and at the end we found out a small company specializing in such services called RuseSoft / www.rusesoft.com. The guys were very helpful and for a few hundred we moved the projects with ALL THE HISTORY! Took them less than 24h from the time we contacted them.
Still wandering why MS does it and OPS Hub is asking for 5K!
Hope that helps!