The nation’s top credit union regulator questioned HarborOne Credit Union’s plans to convert to a bank. Debbie Matz, chairman of the National Credit Union Administration, said she didn’t see how the Brockton credit union’s decision would help its 140,000 customers. HarborOne, the second largest credit union in the state with 15 branches and $1.8 billion in assets, announced plans in February to begin the process of converting from a credit union to a mutually owned co-operative bank.