The documentation here:

http://mms.mongodb.com/help/monitoring/tutorial/update-mms/

makes no mention of the possibility that new versions of the MMS Monitoring might have dependencies that older versions do not.

For example, once upon a time the MMS Monitoring agent ran under Python version 2.4, but at some point it was decided that Python 2.6 would be a minimum language version. As a result, upgrading the agent is not merely a matter of stopping the old and starting the new, except in the debatable sense that someone who reads the sentence about starting the new version might have read the installation instructions (a separate page), and thus noted the new dependencies.

Proposed solution: add a prerequisite step, e.g., "Ensure that agent dependencies are satisfied on the host that will run the new version", and have this step link to the prerequisites, e.g., here:

http://mms.mongodb.com/help/monitoring/tutorial/install-agent-centos-rhel/#prerequisites

For extra credit, the docs might even enumerate the agent version numbers at which the dependencies have changed.