@Baerbel *************
if you change the reason for your extension, it usually starts the day you apply.
Since you would be changing from tourist to Covid, it will likely start when you apply and you will lose any extra days you had.
But if you applied for another Covid extension, that would get added to the end of your existing Covid extension.
This is how it works with pretty much every extension, if you switch types, it starts from the day of application.