Despite what the documentation may or may not say, do not attempt to upgrade your BIGIP without a working, directly connected console cable. We bricked an LTM 6400 by performing the upgrade exactly as the steps stated, but without using a console cable to watch the output.

Technically, a co-worker was doing the upgrade, so I don’t have full details (I did review the CR and steps), but he was doing the “local network install” via crossover cable into the management interface, so there should have been no issues, since the documentation states that’s OK. But we still completely ruined the box. Thank God no impact to the business, due to the HA pair working properly.

Ran the upgrade on the replacement box today with the “console cable” trick from the support guys, saw an error, fixed it, and everything went smoothly. Now to failover from 9.2.3 to 9.4.1 and reap the benefits of administrative domains! (And teach the other IT groups how to put their own stuff in maintenance/disabled mode.)

Edit: due to several searches ending up here:
Connect with a Null Modem cable, set: 9600,8n1. Make sure that your laptop’s serial port is actually working, by testing with a Cisco device or similar – we have 2 laptops that didn’t get any data on the 9.2.3 OS, but did on 9.1.2, changing laptops (or OS in my case) got us back online.