I am hoping that someone here has some experience with this... I have a solution that I am selling through a catalog or two. The solution, which is being sold as both Runtime and bundled with Filemaker, is always being updated and modified (hopefully for the better). The question is how do I update existing installations without the need for importing existing records? Is there an installer available that will update existing installations without overwriting records?

Unfortunately there is no such option/tool at the moment.

I recommend you look into the Separation Model. Updating a solution (UI, Business Logic) without changing the user's data one of the benefits of that design approach. There's a separate topic in the forums dedicated to the Separation model.