How to "upgrade" a site with Composer Manager

My drupal installation worked well with composer manager.

After composer manager was abandoned I replaced composer.json to the newest one but most sites then didn’t work (i have installed drupal manually by downloading and extracting the files by myself). (and there is also the packagist site which was abonded)

If i require all modules via composer they will be installed into a subfolder contrib, but the site still uses the modules in modules folder (if i remove them the sites doesn’t work and it seems that they don’t look for modules in the contrib subfolder …)

If I install drupal new via composer and move the site folders to the the new installation – which itself works correct if i set up a new website – the old sites still dont work …

I get only blank page, also for the site itself as for the admin pages

What can I do?

How can I “migrate” the old sites to the new drupal installation (my first choice, because on the old installation there may be some other strange circumstances) or how can i “upgrade” the old one?

These changes are really boring for non experts …

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