Nope, the best would be to make the primary active and upgrdae the secondary (standby) first to the image that you would like to finally go to, failover and make secondary active and and then upgrade the primary box to the right image, this way you would not lose any traffic and would be up all the time.
Hey Thomas, I just went through it and there are two things in here:
Number one - going by the book (which is wat the engineer recommends you)
Number two - my own personal experience, because there have been situations where in some TAC's I did a lab repro for the upgrdae as other customers were apprehensive about doing it on their production device, I did the way I told you, no issues.
So what he must have told, would definitely have a good reason for it or some experience that he can share, I would say, you can definitely talk to the engineer and he would definitely explain you his reason behind it. Just make sure you have your memory requirements spot on.