After upgrading Nexus, is it possible to roll back from the latest version to an older version?
This depends on two things:
Do you still use dongle based version or are you using license keys? Both versions should have the same licensing system.
Is Nexus running on Mac or Windows?
If you are currently using Nexus 10.1.2 and you want to go back to Nexus 9.5.8 there is no problem as both these versions are running with the same license key system.
However, it will not be possible to go back to Nexus 8.5 for example, as this version ran with a dongle. When working with license keys, you will not have a dongle anymore (unless you are still in the process of switching from dongles to license key files).
Nexus on Mac:
When running Nexus on Mac, you can easily install a Nexus version next to the current Nexus version, as it will never be overwritten. Of course, you will not be able to run both Nexus versions at the same time.
Nexus on Windows:
When running Nexus on Windows, you can only go back to previous versions (major versions) but not to previous revisions (minor versions). So, you can install Nexus 9.5 next to Nexus 10.1 but you cannot install Nexus 10.1.1 next to Nexus 10.1.2. Revision versions will overwrite one another.
A workaround, in case you want to run Nexus 10.1.1 next to Nexus 10.1.2 is to rename your current application folder as: Nexus 10.1.2 to Nexus 10.1.2_OLD.