I'm resurrecting this thread almost a year later. I ended up not doing anything at the time. I'm still running Yosemite, but with all of the comments that I'm seeing about Mohave, I thought I'd better have another look at getting something newer than Yosemite. I'm not sure what's happened since I last tried, but when I go to the Apple Support site, I now see statements to the effect that if the latest MacOS version is not compatible with your existing hardware or software, you may be able to download an older MacOS version. I wonder if they got too many complaints about incompatibility, and finally gave in. So, I clicked on a link to download Sierra, which took me to the App store, and it appears to be downloading Sierra now. I'll report later with my results.
