This is the plan:
- Close the forums.
- Backup the database.
- Upload the new vBulletin 4.2.0 files.
- Upgrade the forums.
- Fix the most urgent problems.
- Open the forums.
- Fix the less important problems.
Preferably, I should upgrade when nobody is watching, but I must also have time for that, so I'll start next Friday 21:00 GMT+1 (my local time). I expect to need an hour for that.