On April 1st 2014, changes to Fannie Mae's guidelines will cause loan interest rates to increase and credit to be evaluated differently. The change in credit evaluation has to do with new tiers of credit, which will be added to the top end of the scale.
Today the main tiers of credit are: less than 620, 620-640, and 740 or more. Each one of these tiers can increase or decrease a borrower's interest rate and closing costs.
With the update, there will be 4 new tiers of credit above 740. Unfortunately, those with higher credit scores will be the most affected - with a rate increase of 1.5% for some loans.
To research further, please see the tables created by Fannie Mae here.