Yes! We advise that the original system continues to be used and is operational until the data migration is signed off as being correct. This means that there is no disruption to services and only when the data is ready and everyone is happy is a final copy of the data taken from the original system and converted. (Remember that the conversion routine is automated using XML schemas as transforms so that it is prepared according to the needs explored in the test loads. This makes it possible to perform repeated loads and checks on the data without manual effort to make changes in the data.
Once the new system is deemed acceptable and everyone is happy using the new system, the old one can be decommissioned. We recommend that the old system is kept in place for a period of a few weeks just to check back on any item in the early weeks of use of the new system. Some clients, knowing the data loads are good and having conducted repeated tests during the conversion turn off the old system at the point of Go Live. Such is the confidence that the migration and conversion process provides.