While the Core Migration process does not impose any restrictions on using your existing (legacy) company data file during or after the migration process, you should avoid doing that unless you have opted for a trial migration. After finalizing your data migration to Core, it is not recommended to continue to use your old software solution and company data file. If you work on it and make changes to it, you will be required to do a fresh migration. When you do that, Core creates a new company file from scratch and all your previous changes in the old Core company will be lost. Check Core Help for details.