Homebuyers with good credit scores will soon encounter a costly surprise: a new federal rule forcing them to pay higher mortgage rates and fees to subsidize people with riskier credit ratings who are also in the market to buy a house. The fee changes will go into effect May 1 as part of the Federal Housing Finance Agency’s push for affordable housing and will affect mortgages originating at private banks across the country. The federally backed home mortgage companies Fannie Mae and Freddie Mac will enact the loan-level price adjustments, or LLPAs.