Please use the search feature in future, would help!

But anyway, every season you're promoted, your players lose a star (reducing your teams overall star quality). The players themselves are still as good as they were previously ... they're just not as good in comparison to your new league level.

For EG. When teams are promoted from the Championship to the EPL, their players are still the same, it's just that compared to RVP and Bale they aren't as quite as good as against Championship opposition. Hence the star rating falling.