What is the best approach of updating existing Craft installs with new features? I'm currently developing a blueprint for all my Craft websites. It contains a set of sections, fields, plugins etc. All configured the way I need for my websites. The websites itself are basically a fork of this blueprint and only has to have a custom template and content and we're good to go.

The number of features in my blueprint will be growing as time passes along. What would be the best approach of updating all Craft installs so that each website (build upon previous blueprint versions) can benefit from the newly developed features.

I could just backup the DB and code from the blueprint and import them into the website, but then I will be losing all content. 

Any idea? 
Thanks in advance!
This approach assumes that the individual websites won't diverge from the original blueprint by, for example, modifying sections and fields to suit their specific requirements.

That seems pretty unlikely.

I'd be inclined to stick with the idea of creating a "blueprint" site, to get new websites up-and-running quickly, but ditch the idea of updating previously-created sites whenever the blueprint is updated.