Thanks for the question Steve. It's a hard thing. Different authors have reported different algorithms. If you apply the algorithms listed in the links below, then the correct answer to the question is "If I have less than 500 rows, when MORE THAN 500 rows are added/changed."
This is an important thing, too, given how much of an impact statistics have on query performance. Of course, I hope everyone has some sort of statistics update strategy in place so the auto updates are limited to large data operations.