Married since September, mom-to-be Spears, 23, has already upgraded to Wedding Ring 2.0, a gift from husband Kevin Federline, 27. "I think it was because he wanted to buy her the diamond ring," a source close to the singer tells PEOPLE about the new 5.5-carat round solitaire. It replaces the five-carat rock she picked out from jewelry designer Cynthia Wolff last year.