The old rule
“More than” was used to refer to countable items. For example, it would have been grammatically accurate to tell your friends that you ate more than 10 bananas in one day but grammatically inaccurate to say that you ate over 10 bananas in one day.
The new rule
The new rule makes it acceptable to use “more than” and “over” interchangeably when referring to numerical quantities, such as dollars or years. For example, “my pet turtle is more than 80 years old” is now synonymous with “my pet turtle is over 80 years old,” and “I earned less than a million dollars this year” is now synonymous with “I earned under a million dollars last year.”