Of course you can. It's just that the script (and "vision") has to be part of it. But there's plenty of extras that can be added only later: getting a professional actor instead of your cousin to do a secondary role, adding that CGI touch to improve ambiance, paying a good compositor to create some custom music, etc.
And tweaking based on research is done all the time: it's called "test screenings".
To me, I would say that when you compare the effort needed to create the "final" print of the film (the one with the CG and professional actors) vs an early test take made with your family, the delta between the cost of the two would be very high.
I feel that a closer metaphor would be that a good treatment and a demo are the "pitch deck" for the real film you want to make, not first versions of it.
Of course you can. It's just that the script (and "vision") has to be part of it. But there's plenty of extras that can be added only later: getting a professional actor instead of your cousin to do a secondary role, adding that CGI touch to improve ambiance, paying a good compositor to create some custom music, etc.
And tweaking based on research is done all the time: it's called "test screenings".