Change the field requirement of the Customer field to Optional. Then hide the field from the Case Form.
Create a N:1 relationship with Account entity and add the Account lookup onto the Case Form. Change the label as Customer.
Now create either a plugin or a workflow to copy the Account you select for the new lookup field to the original Customer field in the background whenever a Case is created or updated.
Follow the same pattern for Opportunity as well.
Thanks
Sachith Chandrasiri
Proposed as answer bySachith VidanageFriday, March 17, 2017 3:09 AM