The marketplace provides CAU RA agent.
When attempting to upgrade, the screen shows a "Target Version" of " Manual Upgrade Required ":
Can the Centralized Agent Upgrade on a 12.3.x system upgrade an RA agent that is 12.1 or 12.2?
Release : 12.3
Component : AUTOMATION ENGINE
12.1 and 12.2 RA agents cannot be upgraded via CAU. The first available version to update the RA agents via CAU is a 12.3.0 agent or above.
The documentation will be enhanced to state that when "Target Version" shows as "Manual Upgrade Required" it means that the agent does not meet one of the requirements (is to old version, etc...)
Make sure that Service Manager is also on version 12.3.0 or above.
More information can be found in the documentation for CAU: