The first time I upgraded the ec2 plugin from v1.42 to v1.43, Jenkins terminated all of my ec2 agents. The following messages appeared in the Jenkins system logs:

maxTotalUses drained - suspending agent i-02dredactedxxxa6a 
Agent i-02dredactedxxxa6a is terminated due to maxTotalUses (0)

This happened even though "Maximum Total Uses" was set to -1 (unlimited) in the Jenkins system configuration.

After manually launching new agents, everything seemed to work fine. It appears that the new version of the plugin does not correctly read the state of the instances created by the previous version, so it thinks that the usage cap was exceeded. If this is the case, v1.43 should at least be flagged in the plugin manager as containing breaking changes.