You could save the information for each client as building blocks with the same names in separate templates and insert the information in your generic template using AutoText fields. Have a UserForm in that master template with the list of your clients as a drop-down. Based on the response to the UserForm, attach the particular client template to the new document and update and unlink the AutoText fields.

• Create a Simple Userform by Graham Mayor, MVP
• Create & Employ a Userform by Greg Maxey

Otherwise, just going with Andrew's suggestion, if you search the vba forum, you will find solutions to change the content of one Content Control based on the choice made in a drop-down content control. Here is one such thread. Either way, some programming will be required.