Ok...so you could do this a couple of ways, but as you mention the word "simple", I'm going to go ahead and skip my API option for now. Let's try and do it with Dynamic content. (do you have Marketo Standard edition or above?) I think that's what's needed for Dynamic content these days.
Either way, you first landing page would be the "generic" landing page in that it is going to be standard/simple for everyone. The second page is what you can make dynamic and in this case you'd need to build a Segmentation in your Lead Databsae for "Customer or Non Customer". Whatever the filter is above that sees if someone is flagged as a customer...use this to build your segmentation.
Now your second page after that first submission is going to look up the individual as you mentioned above, then your form will be the dynamic piece of your second page and you can have 1 form for Customers and a completely separate one for prospects. Or you may even want to do something else for customers/prospects.
Let me know if you think that will work. If not, quite a few other options :)
What type of additional information are you trying to get from these non-customers? Is it information that could be retrieved using a data vendor like ReachForce? Personally, if I started to fill out a form and all of a sudden more fields showed up without a reason (i.e. state showing up after I select country is fine), I'd be likely to abandon it.
Is it essential you gather this information right away? Can it not be done progressively with other content later on down the road?
Michael, yes - that was the first thing I did too. Set up the thank you page based on segmentation. But it turns out this organization doesn't have a "Customer" segment. Bummer!
And when I say the form is simple - I mean the front end of the form itself - ie it just asks for name and email. The backend need not be simple!
I'm looking at this:
Is that the right path?
Jason - the form is sitting on a product "upgrade" page, and the purpose of collecting the information is to match against the list of existing customers (by email address). When an email address match is not made, more information is asked so that the human that will have to find this person in the list of customers will have additional information (like address, date of birth, etc).