[Lynne]

You are talking having the oldest post display first in the thread? vboptions > user registration options > at the very bottom set it Linear - Oldest First. That will set it to Oldest First for all new users. Other users may change it in the User CP. If you want to override everyone's preferences in their User CP, you may run the automatic query in Maintenance > execute mysql query > Thread Display Mode - Linear and then Maintenance > execute mysql query > oldest first

Of course, I will tell anyone to backup their database prior to doing any sort of mass query on it like this.