Addon was the term RealFlight used to begin with for their new content products that added new airplanes and flying fields for use with their current simulator awhile back.. After producing and marketing 5 of them RF decided to give the next one a new name and called it an Expansion Pack. There was some argument for a new name for that type product as they added new functionality beyond just models and flying fields. An example of functionality is the night flying look they came up with. The colorful streaks of light aren't airplane models nor flying fields, it's functionality.

When they came out with the latest version of the basic simulator, the addons would not work with the new versions as there were too many improvements to the functionality of the sim for the addons to work. So they took the time and expense to update each addon to work with the new version. The expansion packs worked with the new sim, so didn't need conversion.

It's not very often that a company does backward compatibility conversions and certainly not for free. Especially a company that is having no problem selling their product. Usually, a company stays alive by making profits and selling new product. This free addon work cost them but not us. It is an encouraging sign of their financial health. And a pretty nice gesture on their part.