JetPack 4.2 used cuDNN 7.3, JetPack 4.2.1 used cuDNN 7.5, and JetPack 4.3 uses cuDNN 7.6. Since I built these with JetPack 4.2.1, PyTorch is expecting to see cuDNN 7.5 or newer on your system (see this code from PyTorch repo). So I would recommend upgrading to the latest JetPack 4.3, it also comes with a number of other upgrades. JetPack 4.2 was the first version for Nano, so there have been a lot of improvements and fixes since then.
Unfortunately yes (so remember to backup any files you wish to save from your Nano first), however JetPack 4.3 is the last JetPack version that you will need to re-flash/wipe your SD card to upgrade to. Once you are on JetPack 4.3, you will simply be able to upgrade to future JetPack releases from the command line (through apt package manager) without needing to re-flash. However to get onto JetPack 4.3, which is the version that enables this new apt-upgrade mechanism, you still need to re-flash.