I seem to remember reading somewhere the locked up seat belt needs to be replaced after an accident when the airbag has been deployed. But can't remember where. You may want to check with a dealer for more info.
ALWAYS replace seat belts that have been loaded in an accident. These are engineered devices, not tested devices. Testing would damage the belt. Only the prototype is tested then all subsequent belts are manufactured to that specification. Once used there is no way to verify that the used belt will meet the specification.
These also should have pyrotechnic tensioners which go off if the airbags go off so you must replace the entire seat belt assembly. the pyrotechnic tensioners aren't cheap but then your life shouldn't be either.