You can use the "known visitor" feature on the form, but then it will not distinguish between known prospects or customers.
So the best is to use dynamic content and have a different form for Customers, one with minimum number of fields; on, even just a button.
You will need to have a segmentation setup first, then you can use it to make the form dynamic. You will need 2 forms, one for the customers and another, longer, one for anyone else.
See here for more insight on how to set this up: Use Dynamic Content in a Landing Page - Marketo Docs - Product Docs
Apologies for the delay. Thanks for the insight!
How does the segmentation work if clients are determined based on an Account Status in Salesforce? This status can change frequently.
Segmentations are self-maintained in Marketo. Each time the field value changes, the segment is recomputed.