Mavericks upgrade to High Sierra?
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.