Sorry for the late reply, was out of town : )
My 24" iMac 9,1 (early 2009) model is:
C2D 2.93GHz
8GB RAM (upgraded from 4GB)
Regular 640GB HD (not SSD)
GT120 256mb
The machine has surprisingly handled High Sierra fairly well and while the process went smoothly overall (following this guide here, thanks @dosdude1 and hope it's ok to link to your page), I did experience a couple of set backs.
One, for some reason I lost the functionality on my full wired keyboard's USB ports. I used to plug USB flash drives or my iPhone/iPad into the keyboard without issues in El Cap, but now I get a low-power warning(image below)
View attachment 741405
So I reverted to using my old Apple wireless k/b to free up a USB spot on the mac for my i-devices. It's not an ideal solution as I miss the numpad but I'll survive.
edit: this seems to have been fixed with the latest patcher update that addresses the USB issue. I'm back to using the full wired keyboard.
Second issue is I lost iTunes wifi sync with my iPhone 7. This one wasn't too big a deal for me as it was spotty in El Cap to begin with so not a huge loss.
Third, updating from 10.13.1 to 10.13.2 was a bit of a headache in that while the update itself went well, afterwards I lost Safari and App Store functionality. On the recommendation from members in the unsupported mac thread, I installed and ran the free program Onyx for mac (ver 3.4.0) and ran the built-in automation script and that has solved the issue.
And finally, the only other issue I seem to be having is the screen brightness setting doesn't seem to stick between sleep cycles. I installed a free app called Brightness Slider from the App Store and that seems to have fixed this and it offers a wider range of brightness as a bonus (my screen can now dim lower than it previously could - nice for night time work).
Oh, one last item but unsure if this is related to High Sierra. My wifi would sometimes drop connection while showing a connected status. Remedy is to turn Off then On my wifi on the mac and it works after that. My ISP did set up a new wireless modem not too long ago so this could be chalked up to that but I'm not entirely sure.
Overall, I'd say it was worth it for me to do this. Obviously I regained iCloud tab syncing but I also was able to upgrade my Pages, Numbers and Keynote to the latest version which now syncs again with their iOS 11 counterparts. The machine is running smooth but with some noticeable lag/slowness. Not something I care too much about as I mainly use this machine for secondary things (Mail, Safari, writing, occasional image editing.... light stuff).
If you have any more Qs, fire away or post in the unsupported mac thread - many very helpful people there.
Sorry for the late response. Thankyou very much for your in-depth and helpful reply. I haven’t had a chance to update yet but really appreciate your input.
Thanks
