BIG-IP : best practice to update HA pair under load

You can do the auto-sync toggling and failovers via iControl (at least the rest interface, I've never tried the SOAP version).

As I recall from some training courses, however, F5 recommended taking a config backup as the preferred method of change backout (which also could be done from iControl). I think that is operationally less risky than what you have outlined here. I have run into a lot of sync issues on my devices (mostly fips related, may not apply to you) so I'd be leery of flipping that auto-sync bit a lot.

Also, when you fail over, you are likely to loose in flight transactions. I'm fighting with that right now; Whereas on a restore from backup should only touch those items you've changed. I have not tested this, of course, so your mileage may vary.