I am planning to migrate my users from GApps hosted account to Client management system. Is it possible to migrate without loss of any data?
You can start the migration after you point the MX Records to Client management system. After you complete the steps mentioned in the Mail Hosting process, and start receiving emails in Client management system, you can schedule migration for users in batches of 10. Since you would have started receiving emails in Client management system, there will not be any loss of data.
The duration of the Email migration will be based on the amount of data you have in your GApps accounts.