I've seen a tag change price but that price would only take effect the next day, as the store would be having a promotion on peaches. I simply asked for a clarification and the tag was rolled back.
On the day price changes are only done to clear perishables that otherwise will end in the trash, like fresh pastries or bread. And such changes can only take effect near to the closing hours of the store. Price variations throughout the day is a crime and stores have been heavily fined for it.