Established SAAS companies usually permit the download or export of your business data. Depending on the company, this feature may or may not be easy to find. After login, try searching the company's help center for desired instructions. The more data exported, the better. Client information, appointment history, and service details will be the most valuable.
Exporting your Customers from Booker
The Customer List can be exported from the Customers Tab. You will end up with an exported csv document with all of your Customers.
Exporting your Booker Appointments
Booker has a How To on exporting your reports into CSV. If you have a report for Appointments you should be able to export into CSV. Remember to export your future appointments as well as your past appointments.