When creating a booking, unallocated reservation, quote etc, you now have the option to display a popup notification when you select a pre-existing customer. 

This popup acts as a warning that if you were to make any changes to the customer details (eg, change the email address) , that this change will apply to all bookings past present and future for this customer.    

If you want to activate/deactivate  this popup , you can set the system parameter  - "Skip confirmation message when selecting existing customer" to "Yes" or "No" in the System Parameters under the "Booking Process Parameters" heading.