One could acquire the highest 3.9.x code easily enough via git, and you have a DB that would be upgraded using higher code ... but ... what about customizations and plugins (addons) of old site? Plugin references would be in DB but you'll not have any code unless one acquires and installs missing plugins manually. (that's if the upgrade doesn't fail do to some quirk in customizations of core code made in the old site ... which you wouldn't have.
And, more importantly, do you have a backup of the moodledata directory for that old site ... minimally filedir in moodledata?
Without moodledata/filedir from old site no internal links to files uploaded to that old moodle will work after upgrading.
Not sure what one would end up with would be usable ... too much re-do to do ... depending. :|
'spirit of sharing', Ken