has upgraded a (much smaller than yours) ACS 4.2 site to OpenACS 4.6.x
via the "run all upgrade scripts on the database" route. (As opposed
to the "clean install of 4.6 and insert all old data" route.) I don't
know if that'd be the best way to go for you, but it worked for us.
It was not
trivial, but a lot of the work may have been Paul
fixing broken old upgrade scripts, etc.