It’s been a long time since I used mine as well – my A06 board is in a box, but my Devterm has had a CM4 for a year or two. Before I made the switch, I managed to get updates working, but it’s been so long I don’t remember the details. I switched away because it was a pain to build things for the A06 when most ARM based stuff is Pi focused these days and I always kept running into things that were different/missing on the A06.
I’m not sure, but I think these threads helped me get a working system It’s unfortunate the OS image is ancient, but it used to be possible to update it with a lot of digging around in the forums, some effort, and some luck. I may have missed a thread here, so if these don’t help and you still want to try, I’m sure there was something on the forum that eventually helped me get it working. Of course, it’s also possible that enough time has passed that something else is broken and hasn’t been discussed here. 
Personally, I was never convinced that the A06 was that much faster than the CM4 anyway, because whenever I tried to run it faster, it got hot enough to throttle itself down. The whole gears thing was somewhat fun, but fiddly. The one known benefit the A06 board has over the CM4 is that the headphone connector also supports microphone input. The CM4 can’t do that, and never will. (And I thnk the same is true with the CM5?) So it might be useful for folks doing audio stuff who don’t want to plug in a USB mic.
Hopefully these links, or some other posts on the forums will help, but A06 is definitely a project to get working, and I felt like it always would be, potentially at every update. Thankfully, the CM4 works as well as any other Pi device, and since it’s a sort of standard platform, there’s a ton of stuff that works with it. Good luck!
It might also be worth keeping an eye out for A04 posts on the forum. Just recently I noticed an A04 owner managed to finally get the HDMI out working. That’s a huge effort that neither Clockwork nor anyone else got working for… how many years? I doubt there will be much new activity from it, but perhaps some folks with dusty and forgotten A04s will dig them out and try the fix, and that could lead to interest in updated images for the A04 and A06 if someone feels compelled to do it. Probably not much hope for that, but it’s not nil.