When umpire change the cricket ball, was the old ball thrown as waste or keep for reuse?
It completely depends :
If any bowler takes 10/5-wicket haul thn they keep that bowl with them as memory.
Other reason is completely depends on ball condition.
I. Ball is stored by ICC rules to replace it for some overs in other international event.
II. If ball condition is not good enough to play thn ball is discarded.