I have been running my Mid-2010 MacPro (well, effectively that, I believe with the components upgraded it was that) for a long time -- it isn't my main machine, but it is my primary macOS machine. Recently I decided to start on an iOS development project but ran into a bit of a roadblock - the latest Xcode releases would not install on my Mojave-based Mac (the latest officially supported OS for that hardware if I am not mistaken). So on Friday I sat out to update my machine, Apple be damned! It turned out to be a (relative) piece of cake, thought I would throw it out here in case anyone else has one in a similar situation. These are still capable machines, and the tower MacPros are still the best (yes, I know the newest are back to it, but I have no desire to drop thousands on a machine...)
Anyway, these individual steps are by no means my own, just cobbled together from other utilities and information out there.
1. Make sure you are on the latest Boot ROM version for your hardware. For me, I was a couple behind so had to update. To do so, I created a fresh OS X install of High Sierra so that I could run the Mojave installer again. Not sure why I didn't have the latest from the first install, but that is another question for another day. Once I booted up this High Sierra drive, I was able to run the Mojave installer (the official one) and let it update the firmware. I did not complete the upgrade, just needed the firmware.
2. Disable SIP in recovery mode (just means running a single command in the recovery mode terminal)
2. Booted back up my original Mojave drive and installed OpenCore. This is a very small boot loader that is 100% removable if you desire to at a future date. To do this, you really just mount the EFI partition and copy some files into it.
3. Reboot and you can then install Big Sur (or Catalina) and the installer lets it go through as if it were supported hardware.
4. Lastly, in OpenCore you have two basic options - hardware acceleration on the GPU or allowing updates. So of course I turned back on the GPU acceleration. This does mean that when Big Sur shows an update I need to flip that off, do the update, and flip back on. But it is a small thing, only takes a second and a reboot.
I was REALLY expecting this to be a process and it turned out to be well documented and straight forward. I didn't want to go the whole hackintosh-community route that involves all kinds of special configurations on boot and all that; this avoids basically all of that. Just wanted to let everyone know it was not bad in case you are in the same boat that I was in... new life out of an old Mac is awesome (and since it wasn't really supported anyway, updating it un-supported isn't a huge deal).
OF COURSE, don't do this to an Indigo server installation. This should seem obvious but I have to mention it so that Jay doesn't have to go find and copy/paste his warning about running on unsupported hardware lest someone actually try this on a home automation server.