for the past few OS upgrades ( since El Capitan) I have installed the new OS on a new separate SSD drive VIA USB cable and then swapped it out after making sure all my software was working with the new OS on the new drive before installing. Keeping the old OS as the working drive on the MacBook .
This keeps the new install separate as a test version until all is good and then (when ready) swapped out the old drive which becomes my "back up" and the MBP has a new fresh SSD with the new OS.
I prefer fresh installs with new OS versions, specially now that there is a new filesystem around aswell. Had some bugs with upgrades before, and I dont mind cleaning up once a year.
I am not consistent. But for Sierra to High Sierra I just downloaded and installed the upgrade. I had a presentation to give this weekend and did not want to have to build everything from scratch.