This will happen "On or about May 1,2012" the letter says.
Also says "Here's what you can expect for now - you'll continue to have the same account features."
I think the operative words are "for now".
My expectation is that the 5% bonus is soon going to be history even though at the end it says
"It's important for you to know that there will be no change to your Earnings, or the way you redeem them. You may continue to use the Card as usual and keep earning towards your next eligible new GM vehicle."
Reminds me of a letter from my employer many years ago to the effect that "We will never change your benefits." Almost before the (removed) crowed three times the pension plan was eliminated. Christmas come and gone?