I believe that the transformers are slightly different but the mainboard and one of the DSP cards should be the same, so you should have spares.

The display / front panel is also the same.

With regards to firmware, the last 'official' version was 2.12, although there was the 2.5b9 version that has the 'Trifield bug'!
You can load either from 2.00 providing you re-type but I seem to recollect from other posts on the forum, you may have problems going back from 2.5b9 to 2.12.
That's not to say, the firmware won't load... There were reports that certain presets did not function as expected.

Good luck,

Steve

Main: 861v6 (LPS), 808v6, UHD722, DSP8000SE Anniversaries, DSP5200.2HC, DSP7200.2, SW1600, Sky Q, Cambridge Audio BD, Roon, M80
Office: 861v4 (LPS), 818v3, 800v4, HD621, M&K S150P, DSP5500HC, M&K MPS2810 sub x2
Son's system: 861v4, HD621, DSP6000.2, DSP5200HC, DSP33
Spare: MCD, 100 Series, 200 Series, 500 Series, F80s, C15s.