We are running a long migration and our developers are using the onprem TFS while the migration is working. In the documentation I see this:
Note: Your source TFS usage is not required to be frozen or put on-hold for the migration. The migration utility can run perfectly fine in the background, and so your team can continue using TFS while the migration is running
Can you tell me how the tool knows about new changesets that have happened during the migration process ? So once it completes do we need to re-run it to grab these ?
While it is running it's just iterating through all changesets from the first to the last, keeping track of which have been processed and which haven't. While you're working (as long as you don't do destructive things, like tf destroy), it will just keep iterating and will probably end up on 8421/8400 at the end (the 21 are the changesets that were added while the migration was running).
Even when the migration finishes and you decide not to jump over directly, you'll be able to restart (only available in the commercial edition of OpsHub) which will continue where the migration left off. Just make sure you keep the original migration records and restart the migration that finished previously. if you start a new migration to "catch up" it'll compare all changesets between source and target again.