I have built a druapl 8 site with 'Standard' installation profile.
How can I switch 'standard' installation profile to 'minimal' in existing site?
Installation profiles contain code to set up a site during installation, that's all they do. They aren't useful to an installed site.
Trying to change the profile after installation could never conceptually do anything. The installation code has already run, and the purpose of the profile has been served at that point.
There is no way to switch you have two solution,
remove standard modules.