This is due to VSIX limitation of Roslyn analyzers that prevents from finding all breaking changes at once.
Have NUnit v2 and want v3?
This little extension aims at helping you transform your existing unit tests from NUnit v2 to v3 of the framework. Here are the main time savers:
ExpectedException and exception related TestCases are automatically transformed to v3
Various assertions and constraints APIs are swapped
Renamings to v3 equivalents
Other problems detected and reported as warnings or errors (no automatic migrations though)
How it works
You normally do it in the following steps:
Install the extension in your VS.
Open the project and go to the Error List.
You should see a list of errors/warnings with ids like NU2M01. This is not the case. You need to open a file first to get it analyzed. And this is troublesome, since you'd like to find all breaking changes automatically. You're forced to make assumptions and this reduces benefits significantly. A better option is to use the nuget package.
Navigate to one and check if a code fix has been provided (in most cases it has).
Transform as much as you can with code fixes. Other adjustments you need to do manually.
Update nunit library to v3.
Compile and run unit tests. Some may fail. At least the automation removed the boilerplate transformations from your shoulder.