I think the question you are looking for is how to apply a feature update with PDQ Deploy.
Look on a computer that has been updated to 1803 for the "1803" temp install files directory.
Copy that directory to your repository.
Create the following step (including copy the entire directory)
$(Repository)\1803\setup.exe /auto upgrade /quiet
Make sure you test this before widespread use. It's worked well for us but might not for everyone.