Can I add fields to an infopath form used as a SharePoint content type w/out losing my existing data for previously completed forms?
Yes, you can add fields to the infopath form and put them into play. Your existing forms will upgrade as long as you don't delete any currently existing fields. I have used this method myself to add to a form as requirements changed. When you publish the form, it will upgrade the forms currently in use either through the site collection or at a farm level if published through Central Administration