MDS stuff is time-consuming enough that I wouldn't try to both swap hardware and upgrade in the same window. I would prefer to lock one in, then go for the second in a separate window.
If you want to swap the hardware first, I would install R81.20 on the new hardware, then downgrade in place to R80.40 with CPUSE.
If you want to upgrade first, I would just upgrade in place to R81.20 with CPUSE, then swap hardware with mds_backup.
If you want to do both in one window, I would still serialize the steps into an upgrade and a hardware swap.
If you're really, really sure that you want to do both at once, the process you listed seems fine to me. Just expect it to take 10+ hours.