Set Custom Form based on the Customer selected on a transaction a via Workflow

Here is how to implement the workflow:

WORKFLOW SUMMARY
-Basic Information-
   Name: SO Change Custom Form
   Record Type: Transaction
   Sub Types: Sales Order
   Execute as Admin: Check
   Release Status: Released
-Initiation-
   Event Based
   On Create: Check
   Trigger Type: Before Record Load

WORKFLOW STATE
-State 1-
1. Set Field Value (Action 1)
    Trigger On: After Field Edit
    Client Fields: Customer
    Condition: Customer = John Doe (or any other Customer you want to be associated with the Custom Form)
    Field: Custom Form
    Value Selection: Custom Sales Order 2 (or any other Custom Sales Order Form you want for that Customer)
2. Set Field Value (Action 2)
    Trigger On: After Field Edit
    Client Fields: Customer
    Condition: Customer != <Customer set on 1st Action>
    Field: Custom Form
    Value Selection: <Other Custom Sales Order Form set on Action 1>

Right after you created the workflow, navigate to Transactions > Sales > Enter Sales Order. Then on the Customer drop-down field, select John Doe (or any other Customer you want to be associated with the Custom Form). The Customer value will remain on the form when it is reloaded but the other field values will be cleared out.