My understanding is that it will temporary upgrade once you approved the action and load in the project. Then for content the user has already approved upgrade will do it on the fly without prompting?
Yes. If you are viewing a channel containing 2018 Revit content within Revit 2019, the user will be prompted to perform a file upgrade. The AVAIL Browser for Revit will temporarily upgrade the file and saves it to a temporary cache on the user's machine. If the user revisits that same file in the channel, they should not be prompted to upgrade again and instead retrieves the file that was cached locally.