I'm going to reveal something that I reckon 96.7% of Mac users don't know about or ever consider:
If you want to try an OS upgrade before committing yourself to it (installing it on your internal drive), TRY IT ON AN EXTERNAL DRIVE FIRST.
You can buy a cheap 2.5" SATA SSD (128gb) for about $20 or so.
Put it into a USB3 2.5" external enclosure.
Then, install the new OS onto the EXTERNAL drive.
Set it up with a new account, or migrate your existing one over.
Migrate apps, too.
Now you can boot and run the new OS, without messing with your existing OS on the internal drive. If you don't like it, or some apps no longer work right, you still have the internal, older OS "unchanged".
At some point, you may decide you ARE "ready to commit" and upgrade the internal.
Then, just repeat the above process, using the external drive for THE NEXT version of the OS that comes along...
One thing:
Although you CAN install modern versions of the OS onto platter-based drives, I wouldn't advise doing so. The Mac OS really needs the speed afforded by an SSD.