Spawn eggs can be used to change spawners.

[commandblockguy]

Spawn eggs created by throwing eggs at mobs can be right-clicked on spawners to change the type. This is
vanilla behavior that is not usually an issue because spawn eggs are unobtainable in survival. This might unbalance the economy on some servers by making it significantly easier to farm mob drops like emeralds from vindicators and gunpowder from witches.
It would make sense if there was an configuration option to disable this in survival mode or make it so that mobs spawned from changed spawners do not drop items to prevent farming, possibly on a per-mob basis.