it is possible to migrate open incidents but does take a little work and will require a sql guru and a little workflow knowledge. My company did not want to run with two systems up. so we can up with the following.
We created a new sql table that contained the IM#, Title, Description, Category, Affected User, contacts,comments, assigned team, adn new IM#
We populated the new table with all the open incidents. I then created a workflow to read each row and created an incident off the data and wrote back the new incident number for verification. We successfully migrated 1000+ incidents. So it is possible to migrate open incidents. the other thing i added was the old IM# into a comment so the Technician could refer back to the old system just in case. I recommend trying it on a test system first it took a couple of runs to get the workflow down.