The update did some very interesting things to the Yahoo account. It is possible it did not transition well. My recommendation is to go to Settings > Mail, Contacts, Calendars, choose your Yahoo account and delete it. Be sure to select to delete/remove data on the iPad. That will prevent duplication.
Visit the Mail, Calendar, and Notes (if you used this) apps to make sure the data is gone.
Now add the account again. If you get keep/merge options, it's always safest to chose merge. You can always weed out duplicates later.
If you had not noticed before, I think you will be pleasantly surprised at the new syncing options.