Heh! Graham, I've been playing keys since my dad's 7' grand and Hammond w/ Twin Leslies for about 48 years, so not too different in age (you can tell I'm 50 in my latest YouTube vids, no? I haven't uploaded any since hitting 51, working on a few though)
Yamaha has 4 basic OS/interfaces for arrangers (The workstations were produced by a completely separate arm from the "home keys" division, and thus the completely different OS's on their workstations like the Motifs.)
The entry level interface, which is used in all the PSRE series, all DGX prior to the 670, and even CVP's up to the CVP201/202, is their clunkiest.
The 2nd level up, which was previously used on the PSRs670, and now the sx600 and DGX670, is the same as their previous top of the line arrangers with the 10 face buttons (A through J, which made navigation a breeze) but without the 10 face buttons! Instead cutting costs by using 4 cursor buttons to get around. This current DGX/sx600 interface isn't as easy to use as the old top of the line 10 face button interface or the new touch screen interface, but it's miles ahead of the entry level interface that all the previous DGX (and current PSR-E series) employed.
The 3rd level up uses 10 face buttons A through J which means it works similarly to touch screen, but instead of a touch screen you pressed the button next to the desired menu item. All Tyros, PSR-S series, PSR2000/2100, PSR3000, CVP309/305/ up to 505/509 all used this; some of the entry level CVP (301-701) also used a harder to see variant where the screen was so small, that the buttons weren't next to the on-screen menu items, but instead had sloped lines drawn to point to where you were accessing on screen.
And the current top level touch screen interfaces, which is easy to navigate but ironically harder to edit with (which is why I kept both of my 10 face button interface keyboards) is universal across Genos/PSRsx and there's a slightly easier to use variant on the touch screen CVP's. (slightly easier because when you turn a layer off, it vanishes from screen so you can clearly see which sounds are active at a glance).
The point is, trying out a PSRE/old DGX will give you a new appreciation for the DGX670's 'level 2' interface!
Mark