We have a legacy Mac in our inventory with Mavericks installed. Just looking for advice from anyone who has done an OS upgrade to High Sierra. Does it work well or is it best to just leave it as-is and keep it cleaned up as much as possible? If upgrading is an option, what were any hurdles that we can potentially avoid? It's a pretty vanilla system other than being a bit on the older side.
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If it supports High Sierra, yes you should install High Sierra on it. I have a Legacy mac that only supports up to Mac OS 9.1, but I guess "Legacy" is a relative term.
"The latest update ruined my Mac" horror stories are all over the internet, but they are the exceptions, not the rules. And most of them are just existing problems that were exacerbated by the process of upgrading the OS, not caused by it. Explaining to someone that knows nothing about technology, that their hard drive has a ton of bad blocks, but they just so happened to not be using any of them until they installed a new OS, usually doesn't go well. So it just turns into "the new OS slowed my computer down, thus I'm now never going to upgrade".
Not upgrading causes you to miss so many bug fixes, speed improvements, optimizations, new features, compatibility with new software. There is rarely a case where it makes sense to keep running an older operating system when a Mac supports a newer one.
Define "Legacy Mac" we will need the model to make an educated guess.
I have an older iMac that (just barely) supported Sierra, but once Sierra was installed it became so sluggish as to be practically unusable. So I wiped it and set it back to Snow Leopard. It is now running at an acceptable and usable speed.
You could use an external drive and make an image of the boot volume, then upgrade and see how it feels. If the new OS does not work well on the Mac then it should be simple to wipe the drive and image it back to the internal drive.
You have then only lost a little time and no data. just depends on how valuable your time is.
Getting your Apple products updated are always a must. As it prevents you from any security risk and unsoomthness of apps.
But first, you can check: https://support.apple.com/en-us/HT201475 to see if your Mac is compatible with the update. If it is, then I highly recommend you to update it.