Key CD is needed if you wish to go from V17.1 (or earlier) to V18 (or later)

To avoid the key CD issue, downgrade to anything between V18 and V21
(old EPROM memory layout)

Then reload >V22 (new EPROM memory layout, 3-1/63 software)

>V22 means any OS CD which is V22 or higher - they all contain the same 3-1/63 OS version for the Mk3.

The goal of the exercise is to force an entire new flash memory load, by going to the old memory map (<=V21) and then to the new memory map (>=V22)