Just an added note: You can find L4T version via “head -n 1 /etc/nv_tegra_release”. When a Jetson is flashed using JetPack/SDK Manager, it is actually L4T which gets flashed (which is in turn just Ubuntu with added NVIDIA drivers). Each L4T release has some incompatible software versus other L4T releases, and thus you cannot mix two releases. In this case it appears the operating system kernel differs between those two releases, and thus you’d have to flash to the newer release if you wish to use that kernel, but chances are there are alternatives.
If you were to then go to a release such as R32.3.1, then you’d find the JetPack/SDKM release which flashes that particular L4T release. However, you can use the most recent version 4.x of JetPack/SDKM, and start with: sdkmanager --archivedversions
(this will allow you to select an older L4T release, though I’m not sure if it goes back that far)