Upgrading is basically an update to the Lightroom database. The upgrade process adds additional fields to the database, this means that LR4 can't read the LR5 database ( lrcat ). They'll also be some updates to the programme itself but the original images are not touched. So even if it all goes belly up ( Very unlikely) your original images are fine. ensure you backup your catalogue before upgrading, and also click the check box "optimise database" in the back up dialogue box. This ensures the database is in good order when it's updated