Don't change to the app version.
I did. Then my phone broke.
Easy, I thought, I'll just order a new key fob... But low and behold, you need the old authenticator to activate the new one. Which was on my phone. Which was broken.
It took ages on the phone, and another letter before I'd got the new fob working again. Grr.
(It's probably sensible that it's not easy to replace the authenticator, but it was posted to my house, which is deemed acceptable for my bank card and its associated pin)