I can not choose which version to install, it install the latest version packed for debian. Currently its 9.3.16
The docuntation says that upgrading minor version is just "replacing the executables", so there has to be a way to save them on the side for roll back.
I see that the binaries involved are located under "/usr/lib/postgresql" - in directories matching up with the version-cluster naming scheme the the apt packages employ. In theory backing up and restoring the relevant directories under there should work. This would only work for minor version switching.
My bad - its not cluster specific ... which makes sense as there is no way to update just individual clusters.