If you're using source control it should be pretty easy to get a delta on what has changed. If not then don't regenerate the code, simply add the fields manually to your existing class and rebuild. If you want to create custom methods or extend the
Provider class, then simply add those customizations to the class you need keeping in mind to not regenerate that code from the Kentico UI.
Get in a standard practice of creating your classes in the UI then gerating your code only 1 time to get the majority of the base code written for you. This is our common practice. Then you know once the code is generated, you simply make your changes to the UI and/or code without generating the code again.