NimbostratusHA fail over, in later releases of TMOS, is favoring Network Failover. Additionally Serial failover only works on a pair of devices. In v11.x there is support for Device Service Clusters which scales beyond 2 devices.
My point being that if you upgrade your 10.2.4 unit to a current release of code the chances are that you may not leverage Serial Failover.
In anycase, I would use the guidelines from: SOL11215: Upgrading an active/standby BIG-IP high availability pair :
http://support.f5.com/kb/en-us/solutions/public/11000/200/sol11215.html
Section: Upgrading an active/standby BIG-IP high availability pair configured with serial cable failover
Ie, ignoring the upgrade part, but taking the guidance on disabling the respective HA triggers, and building the pair up (serial and network) and re-enabling the HA triggers
