There are a few rules and recommendations to take into consideration when migrating data from one system to another:
It's important that you stop using your original system before we start the full migration. You can continue working in the system for the purpose of the sample import(s)
You are OK to use the new system while the migration is still in progress. However, while the migration is in progress, you should not delete or merge any records.
We always recommend that you schedule the migration for a time when your team is out of the office to minimize disruption to your business.