You are right @fussl!
For instance, this is a fragment from the first boot script:
if config_name == "V5" and os.environ.get('RBPI_VERSION_NUMBER') == '5':
config_file = f"zynthian_envars_{config_name}_pi5.sh"
else:
config_file = f"zynthian_envars_{config_name}.sh"
As you can see, the config file template is different depending of your RBPi version.
If some of you want to re-use a SD-card installation after changing the RBPi version, you should “reset” the card so the first boot process is run again. This would do the task:
set_first_boot.sh
All the best,