I've bought several 6 star players and had them drop to a 5 star, and pop back up to 6 stars again.
I've also bought many many 5 star players, but have never managed to get them to go 6 star.
Does that mean they cannot?
Ie: I buy a 5 star at the start of the season, plays every game (sometimes 3 in one day), never ever goes up to 6 star, but other players I paid 50 tokens for, go down and have gone up again.
Cheers, best game in the world by the way...