In my experience, every time the app updates it wipes the downloaded maps off the device and they have to be downloaded again. I only have two maps (and I won't be buying any more), but if you've got dozens of maps this would be a real pain, especially if you only discovered that you didn't have the maps when on the hill or if the app auto-updated on 4g and then you had to knacker your bandwidth to get the maps. When I emailed OS about this and pointed out that most other apps manage to update without losing data they said that this wasn't a problem because WiFi's free.
I've only got Premium OS membership cos it came free with a magazine subscription. They can F**k off at renewal time.