During a ZDU when agents don't see a CP with the target version they will reconnect to a CP with the initial version. This can happen in a situation where Net Areas are defined for only one CP.
When clicking the Reconnect Agents button, the CP that does not see a target version disconnect and therefore reconnects to the same CP with the initial version.
During ZDU all agents will need to have a target version CP in the [CP_LIST] section of their ini file that is available.
The uc4config.xml file on Tomcat will also need to have a IP address of a CP that is already on the target version to properly reconnect.
Alternatively, the agent has to be stopped and only restarted after the ZDU is finalized.