I recently fazed out my 3900s to the i2800s. Per F5 support, I added the i2800s to the existing 3900 cluster. I am running 12.1.2. When the 3900s were installed, they used the serial interface for the configsync. So make sure you define the network configsync link and the failover network across all the devices in the cluster. You can then force offline the 3900s, and let the new units take over the sessions. Once everything is comfortably running on the new units, you can remove them from the Device groups and Traffic groups.