The four people named by Ford have already produced sworn statements that they were not there, that the party did not happen, blah-blah-blah. They cannot change those statements without being under penalty of perjury.
Technically they can. The statements were mostly along the lines of "I don't remember". They can start remembering and that not necessarily perjury. Or for example if the FBI uncovers more specific details such as the date/location/etc and asks more specific questions then witnesses can provide more specific answers without contradicting their previous statements made about a more vague allegation.