The EVs' effect don't change according to this-
If the Pokemon has been fully ev trained then there is no point in going past level 50, stat wise. Whether the Pokemon is level 50, 75, 100, 83 or any other level 50+ the stats will always be scaled down to its level 50 base stats where evs and ivs then take effect.
From Game FAQs. And it is pretty reliable.
But the stats will change to what the were like at level 50. But the EVs' effect will remain the same.