Can USAA approve my VA loan and then decline it a month later?

I was approved for a 100k VA loan mortgage. I received my pre-approval email from USAA telling me the amount I was approved for and it also said the date to which the amount was good till. After calling USAA telling them that I found a home and want to move forward with the purchase, they tell me that I am no longer approved because my credit score is 630 and the minimum is 620. I have read that when applying for a VA Loan, your credit score is not a deciding factor by the last 12 months of consecutive payments are. I have made all my payments on time for the past 48 months. USAA told me that there was a policy that took effect shortly after I was approved and that I was not grandfathered into that category. The new policy now looks at the credit score of the borrower. Can USAA do this to me? Should I apply for a VA loan at another bank?

I was approved December 17, 2011
Below is the confirmation email that USAA sent to me.

Thank you for choosing USAA Federal Savings Bank for your mortgage financing needs.

Congratulations! You’ve been pre–approved for a mortgage loan from USAA Federal Savings Bank. Your mortgage loan pre–approval for a $ 100000.00 loan amount in XXX County, TX will be available until March 14, 2012.

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