If you promote don't be surprised your players loose a star. At level one a 3* has quality 10-14, at level 2 quality 15-19, level 3 quality 20-24, ... . The players loose also part of their market value since they become a year older (it is always the same: you train them during the season which increases their quality and their market value - at new season the market value drops, you train them, ....