Transition a contact from one stream to another

Hi,

I'm making several streams of emails. A person receives a stream based on two criteria:

• whether they are a prospect or a customer
• their product interest or product purchase

So the person might be:

Prospect interested in one of these:

1. product 1
2. product 2
3. product 3

Customer who bought one of these:

1. product 1
2. product 2
3. product 3

Prospects come into Marketo through a form.

Customers are fed into through a feed from a sales management system, and identified as customers with a checkbox in their record.

When a new customer is already in Marketo as a prospect, I want marketo to transition them from the prospect email stream, into the customer email stream for their relevant product choice.

What's the simplest way to set this up?

Thanks!