dear graham,
i did not find upgradation smooth even from 2.3.1 to 2.4. I upgraded as per upgradation notes, which are very simple steps as you think when u read them BUT life will be challenging when u do it yourself.
Smooth moodle update , till time i found from my own exp and posts in this scetion, is either a LUCK or u need to have great patience to do so. Now i am thinking of removing 2.3.1 and install fresh copy of 2.4 and then import all courses and users. This is extremely time consuming job if you have high no of courses. But i do not see any other otiion. AFTER UPGRADING FROM 2.3.1 TO 2.4, I LOGGED IN AS ADMIN AND FOUND SITE CONTROL LINKS ARE INACTIVE. I have posted it on this forum but my issue is still unresolved.
Another thing is u need to have server admin privillages if something goes wrong, which may not be the case always. So u need to depend upon server administrater of your site to try our different ways to get moodle back in working condition.
I do not want to discourage you but just shared my experience during upgradation.
All the best,
Nilesh Kulkarni