We would like to use the free OpsHub tool to migrate from TFS to VSTS but there is a 2500 change set limit. We have almost 5000 change sets from 4 years of check-ins. Is it possible to delete the oldest 1/2 of the change sets to allow use of this tool?
No. Changesets are immutable. If you could delete changesets, you could easily disrupt your source control history.