Mortgage Rates

With the 10 year Treasury yield down in the two and a quarter range mortgage rates have falling to a crazy low, 30 year fixed is well below four and half.  Great time to refinance for everybody the interesting situation that arises is now the US has millions of mortgages locked in at say 4.5% for thirty years, or more appropriately said Fannie Mae and Freddie Mac will have all these loans on the books, since they own 90% of the mortgages in America.  Now the interesting part is if the Fed ever starts to raise interest rates in the future what does Fannie do with the loans they have that are paying less then say the 10 year bond?  Right now everything is fine but what happens if we ever need to go back to a more "normal" rate.  Fannie will have a book full of loans paying 4.5% in a 5% world.