Thanks to a chat with @tamalien, we updated the text for clarification:
If you have MSFS content installed completely on a custom path, you will have a much lower chance that a full re-install will be necessary when the update is officially released. However, if your sim content is installed in the default path selected by the in game content manager, you may have to reinstall the whole sim when moving from the test build to the public build. We recommend that those who wish to test with us ensure your sim content is located in a custom path before joining the flight to avoid full reinstallations in the future (Details on how to do this can be found here) . The best set up would be for your store app to live on the default C: drive and sim content on a custom drive.