Near the end of the commercial availability of the ZX Spectrum , a lad I went to school with wrote in asking Your Sinclair magazine if there was any way he could upgrade his 48k+ to make it a 128k.
The magazine's reply was no, he couldn't, it wasn't possible.
I had always believed that was the case so, I was amazed recently, while looking through magazine PDFs from a couple of years earlier, that in Crash 78 (July 1990) there was an advert for the Videovault 128k upgrade.
The upgrade not only offered more RAM but sound through the TV (the advert doesn't say whether that is through an AY PSG chip or not), Kempston (Atari standard) joystick port, parallel printer port, and - presumably for rubber key 48k owners - a reset button.
The advert goes on to say that the Spectrum owner should send their computer to VideoVault at their address in Hadfield Cheshire, in order that they could make the necessary modifications "to the board".
Of course, this upgrade may not have been available a couple of years down the line, but it certainly seems to suggest that - at one time at least - the upgrade to 128k was possible, if a little fiddly and somewhat expensive.
Thanks for reading,