Well - that was a morning wasted!
Got everything set up for the swap. Followed the advice in the link above & took screenshots of every relevant page of the VM SH setup, read the procedure several times from several sources, got the new router sited and cabled up, switched the VM hub to modem mode and...... nothing.
After connecting the new router, powering up and connecting the 3 ethernet cables (one to MacPro, one to a switch with various wifi hubs and another to a HDD dock), I wasn't able to log in to either the new router (192.168.0.1) or even the VM SH (now theoretically changed to 192.168.100.1). Certainly no wifi.
I then spent a while swapping out cables, rebooting things in different orders and finally unplugging the new router and connecting the MacPro directly to the Superhub (bottom ethernet port, which flashed appropriately). Nothing, zilch.
A phone call to VM CS wasn't revealing, although she was able to return the SH to its original state (something I of course had been unable to do myself) so at least I'm back to square one, but no explanation as to what might have been the problem. I am being sent a new SH - the newer version (although from what I read that can be even more problematic).
Oh well, I tried.