My standard advice:
If you are quite happy "where you are now", why not "stay there" a while longer?
If it ain't broke, don't "fix" it.
That said, if you want to try High Sierra anyway, there is one EXTREMELY IMPORTANT thing you should do first:
Get an external drive (USB is fine), get CarbonCopyCloner, and clone the contents of your current setup to the external drive BEFORE you do the upgrade.
A TIME MACHINE BACKUP IS NOT GOOD ENOUGH.
With a bootable cloned backup, if things go wrong, it will be child's play to:
a. boot from the cloned backup
b. erase the failed install
c. RE-clone the cloned backup BACK TO the internal drive.
You'll then be "right back where you started from", as if you'd never left.
If you DON'T do this, and things go wrong, you will be "up the creek with a tiny paddle", and it won't be easy to get back.
Next thing:
I recommend that you create a bootable USB flash drive installer (of High Sierra) and do the install that way.
You'll need:
a. The installer (download it but DON'T run it after the download)
b. A USB flashdrive 8gb or 16gb
c. The free little app "Boot Buddy" from here:
Boot Buddy – sqwarq
Use BB to create the installer on the flash drive -- takes only a few clicks of the mouse.
Then...
Boot from the flash drive, "aim" the installer at the internal drive, and install that way.
But again -- you want that cloned backup close by, in case things don't go as planned!
BTW, CarbonCopyCloner (or SuperDuper, which works well, too) is FREE to download and use for 30 days. Long enough to do the job.