Quoted from the http://tidbits.com/article/17633: "If you're running macOS 10.12 Sierra or earlier, and do not want to upgrade to 10.13 High Sierra right now, be careful because Apple has started pushing High Sierra to older Macs and making it all too easy to upgrade inadvertently." [...]
(Score: 2) by theluggage on Tuesday November 21 2017, @02:43PM
No. I'm not overjoyed by the way they casually promote major OS updates to possibly non-tech-savvy users (we all know who has to pick up the pieces) but the way Apple do it it is only going to catch the compulsive clickers (in which case the installation will probably be blocked by all the malware and "purify my Mac" apps they've already installed).
Its pretty clear what the "Upgrade" button is going to do. None of this MS-style "Don't click 'Yes I Do Don't Want It' if you don't wish to reject the opportunity to not decline this upgrade" malarkey.