Hey Gary,
My upgrades went little-by-little with my MP 2009, 4,1 machine which does max-out at El Capitan...
First it was hard drives, them RAM maxed out, a second-hand, flashed Radeon 7950, SSDs, then a couple of years ago I swapped-out the 4-core 2.66GHZ for a 6-core 3.46 Xeon...
Both the Radeon and the 6-core were from eBay. The Radeon was simply a matter of plug-and-play and install a driver but the 6-core was a little more but quite easy. First thing was to update/flash the EPROM to update the system to 5,1. The vendor I used had all the instructions and steps as part of the update and sent the link to get me started. The Xeon wasn't too bad to replace and the kit included the tools needed to disassemble the processor card heatsink and I think a spudger to pry out the old CPU along with thermal paste.
That update didn't take too long and was quite easy.
In the end, I was able to update to Mojave and ACR ran more threads simultaneously as it processed my raw files, so it worked out well.
Even though I'm working on files that now regularly tip the scale around 400-500 MB in size and larger, overall speed still seems quite good to me for my normal workflow and processing sometimes several hundred files at a time for an occasional wedding or other large event.
I would be happy to continue working on this system and software for years to come but know the writing is on the wall and in the next couple of years, I'll need to upgrade to one of the newer RISC-based Mx systems and newer MacOS versions. Even so, I'll still be running this system for the foreseeable future since I still have totally functional though now obsolete software I just can't see paying for upgrades/monthly license fees, etc.
I'll most likely need to have one system for photo processing and this system for everything else...unless I win the lottery ;-)
So to make your older system Mojave compatible will in the very least require you to find the instructions and EPROM installer to modify your system to 5,1 and then you'll need to find an installer for Mojave.
I kept on High Sierra as long as I could knowing my system was quite stable. BUT Adobe updated PS/ACR with features I really wanted so I bought a newer and larger SSD, cloned my old system onto it, did the update and found that the upgrade went quite well and that I had both few issues and little software I needed to upgrade.
By doing my last system upgrade on a new drive, it gave me a fast and easy downgrade path just in case. Luckily, the upgrade went easily and I was up and going in just a couple of days...