I've always changed it, and kept it in case they want it sending back - I'm fortunate enough to have a little stockpile of them as they will send a new one even if 12 days in for example, so with the 2 I get per month on prescription, I've got a few spares! I've had to reinstall the app a couple of times as it was losing connection to healthcare team ("Connected Devices" it was all of a sudden greyed out), and another time was not pulling through any of the data from my Insulin Smart Pens and it is annoying that you lose the data on the phone, but you can still get all of it from Librevie