It is possible to start prototyping in blueprints, then “extract” the C++ sourcecode of the blueprint so the blueprint can be dropped and continue in C++ only?
No, after you prototype somethin the c++ locks until NG+. But seriously - of course it’s possible, and it’s a way a lot of developers work, it’s faster to prototype something in blueprints, and if you are using source code version you can see code of every node, so you can easily check out how it’s done.