How will you do the change? Visually is it easily possible to tell them apart?

This makes me thing to the Peugeot 104, launched as a 4-door sedan:
https://en.wikipedia.org/wiki/File:Peugeot_104_1974.jpg
and changed to hatchback in 1976:
https://en.wikipedia.org/wiki/File:Peugeot_104_GL_(15436439521).jpg
Not easy to tell them apart