Thanks to CPCwiki I was able to upgrade my 464plus to 6128plus spec! Yay!
I did mine in a slightly different way from normal so I only thought it fair to contribute back to the community. I made a video guide/build log showing all the steps -
https://youtu.be/uKaETSxEVMo
(most of you can probably skip the first 4 mins or so)
I also edited the upgrade page with some extra info from issues 90 and 91 of Amstrad Action, which has slightly wrong info but is helpful nonetheless : https://www.cpcwiki.eu/index.php/464Plus_Conversion
Since the scans available elsewhere of AA aren't very good quality I got my parents to dig my old copies out of their attic and they very kindly scanned the relevant articles for me : https://www.cpcwiki.eu/index.php/Amstrad_Action,_Issue_90,_Mar_1993 and https://www.cpcwiki.eu/index.php/Amstrad_Action,_Issue_91,_Apr_1993