Thank you very kindly! I’ve done what was suggested.
(I do intend to upgrade, but I just want to choose when. If I’m careless and lose the chance for a free upgrade, well, that’s my own darn fault, but at least the choice was mine!)
If and when you do decide to upgrade, get yourself a big removable hard drive and use something like CloneZillato back up an image of your entire system. Upgrade problems are pretty rare, but their rarity is no comfort to those that they do actually hit.
Download and install GWX Control Panel. It stops all Windows 10 updates and nags – and has a button to click when/if you decide to got ahead and install Windows 10.