How many times after importing a solution with Plugins, we have either errors because of versions, missing steps, the system complaining of something, i think the next few lines will help to understand what happens in the back-end when importing a solution with Plugins (changed code or even less or more steps).
Plug-in assemblies can be versioned using a number format of major.minor.build.revision defined in the Assembly.info file of the Microsoft Visual Studio project. Depending on what part of the assembly version number is changed in a newer solution, the following behavior applies when an existing solution is updated through import.
The build or revision assembly version number is changed.
This is considered an in-place upgrade. The older version of the assembly is removed when the solution containing the updated assembly is imported. Any pre-existing steps from the older solution are automatically changed to refer to the newer version of the assembly.
The major or minor assembly version number, except for the build or revision numbers, is changed.
When an updated solution containing the revised assembly is imported, the assembly is considered a completely different assembly than the previous version of that assembly in the existing solution. Plug-in registration steps in the existing solution will continue to refer to the previous version of the assembly. If you want existing plug-in registration steps for the previous assembly to point to the revised assembly, you will need to use the Plug-in Registration tool to manually change the step configuration to refer to the revised assembly type. This should be done before exporting the updated assembly into a solution for later import.
Hope it helps.