I can only find three (3) differences on the back:

1) On the top line the dashes changed to dots:

2) On the second line from the bottom again the dashes changed to dots:

3) On the second line from the bottom the ™ changed to ®:

The first card from Wynn:

Here's a picture I took of Encore being built, taken from a room at Wynn Las Vegas in 2007: