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.
If so, BEFORE you attempt the upgrade, use either CarbonCopyCloner or SuperDuper to create a bootable cloned backup of your existing OS (internal drive). Both CCC and SD are free to download and use for 30 days.
If you do this, and find that the upgrade "goes wrong", you can then boot from the backup and reinitialize and restore your "old OS" from the backup BACK TO the internal drive.
It shouldn't delete anything that you installed/created yourself (but as noted, backups are essential), but there are a few Apple-provided things that will vanish: Google Talk support in Messages, telnet, and the translation widget. Hopefully you don't use any of these!