try using the same (approx) sized star type bit, it should just pick up the corners of the old hex shape, you might have to give it a bit of gentle persuasion to get it in but this has worked for me before.
this was exactly what i was going to say!! . . . ..if you can get one slightly bigger, thats a tight fit and tap it in with a hammer thats even better!!!
make sure u turn the ratchet or screwdriver dead sraight . . .this is important so u dont twist the bit out
( in that i mean if your using a driver make sure its pointing straight down . . and if its a ratchet it must be at a 90 degree angle!!!!!!!!!)<<< this is really IMPORTANT
plenty of downward force and turn slowly
. . .remember slowly . . ..slowly catches monkey . . . .or sum s**t like that!!!
*** sorry thought u were trying to get them out!! still applys to getting them in . . . . . .but you should really take them out and replace them. . . .if there bad now and you tighten them up ur probably gonna make them worse and u'lll never get them out!!!!!