"Can we have, or is there already, a dummies guide to installing over the top of a 32bit installation while retaining /home and settings?"
not a guide but some ideas
it depends on what you want to save from your home folder, if it is config files, personal files or entire folders of mails and similar stuff
i find easier to copy the contents of your home folder to another place(external hard disk or similar device), wipe hard disk, install and then copy files back to new home folder on the new os
if any folder or file permission problem happens, you can find help here to fix that
another option is install it over the 32 bit version if the home folder is a separated partition, not deleting or formating it but using the other partitions for the new os, system should leave home untouched and will be used on the next boot
if you don't have a separated home folder, then the first option is the only option i think