Thanks for your reply. I think I did find a solution, which is a hybrid of ideas I've read here. Since the messages had all already been uploaded to iCloud, I went to my iCloud account using Safari, and created an empty folder. I then moved all the messages from the folder I wanted to re-sort, and moved all the messages into it. BUT FIRST, change the sort order in that folder so the oldest is listed first. It apears that when I moved all the messages into this new folder, iCloud puts a new time stamp on it. SInce you want the oldest message to have the oldest time stamp, you have to sort the folder with the oldest first because iCloud moves the items starting at the top of the list. So the newest messages at the bottom of the list will be moved last, thus having the most recent time stamp. For some reason, iCould marks some or all of them as unread after you move them, so if you don't want them to display that way, select all the messages in the folder and under the actions drop down (the gear icon), mark them as read. If you don't do this before going to the iPad, they will show as unread there, and it's much harder to mark them as read on the iPad. I then went to the iPad and opened the corresponding folder, and it loaded the messages in the proper order, with the newest listed first.
For some reason, however, regardless of what setting I choose for how many messages the iPad should show (50, 100, 200, etc.) it displays all of them. Of course it won't actually load them unless you scroll way down in the list to show the older messages. But I can live with that.
You can then go back to iCloud (on Safari) and delete the folder you emptied out for the move, and change the name of the folder you moved the messages into back to the original name. This is faster than actually moving all the messages again.