I think 500 packs are too few for such upgrade unless a 7 or 8 star player trains at almost the same speed as a 6 star. Also for a 4* to require only 50 packs, he should always win 2,6 times the normal amount of xp on normal training which isn't the case even if you train after a match. These are my findings with a medium training ground, anyway. Can't say for higher lvl training facilities.