I cannot see that it would be a problem. I just upgraded my MBP from Mavericks to High Sierra and nothing was lost in the transition. You might find that certain applications may not be compatible after the upgrade, so you need to check that before proceeding. Also, you need to backup everything using more than one process. What I mean by that, for example……I have two Time Machine destinations on my network, plus I also perform two separate Carbon Copy Clones of my working drive. I have recently helped (not much help really required) a couple of friends upgrade to High Sierra. In all cases the upgrade was totally seamless. Everything that was there before the upgrade was there after the upgrade.