In my misbegotten youth....
I purchased an upgrade from Vista home OEM to windows 8 retail via Microsoft for the grand total of 25 whole English pounds (when they were desperate to increase windows 8 uptake). The key has been used subsequently to upgrade to windows 8.1 then 10 and since then the "downgrade" to windows 11 - going on 15 years of MS OS's now (all those other OEM licenses from machine upgrades have been wasted). This is linked to my MS account so with luck I will get a few more MS Os's out of it.....
Now who remembers calling the MS activation number for XP numerous times after another wipe to get the machine working again.....