One of the themes of .NET 6 is writing less boilerplate code and helping developers focus on their code instead of ceremony coming with the programming language and the framework. The file scoped namespaces feature is one of C# 10 features that we can consider under that theme.
Visual Studio offers a fix to convert a block-scoped namespace to a file scoped namespace. However, you can only apply it for the current document. Therefore, it's not possible to fix all occurrences in a project or a solution.
On the other hand, it is possible to convert a file scoped namespace to a block-scoped namespace and fix all occurrences in a project or a solution.
This is how the refactoring/fix pair works. The default namespace declaration preference is block scoped. So, Visual Studio provides refactoring for converting file scoped namespaces to block-scoped namespaces.
There are a couple of ways to change the default namespace declaration preference.
.editorconfig File
If your project or solution contains a .editorconfig file, you can add the following line and make the file scoped namespaces the default preference.
csharp_style_namespace_declarations = file_scoped
After adding the line, Visual Studio will offer the "Convert to file-scoped namespace" refactoring and you can fix all occurrences in a project or a solution.
Changing Visual Studio Code Style Preference
If your project doesn't contain a .editorconfig file or you look for an alternative way, you can change the code-style preference in Visual Studio globally so that namespace preference applies to all projects that you work on. To change the preference globally, open the Options window(Tools=>Options). Navigate to the C# code style preferences by clicking Text Editor=>C#=>Code Style and then change the related preference.
I hope this post helps you to adapt to the new features coming with C# 10.