They've marked white before on a HMDA app.
There is no provision in Reg. C to use previously collected information. That allowance is only for Reg. B collection purposes. For Reg. C you report what the applicant provides, and it doesn't matter if they provide different information with every application submitted. You also report information not provided in a mail or telephone request if the information was not provided by the applicant regardless if you have previous information that was reported on a "different" application. Reg. C is an application regulation and you report the applicable information for the application submitted, not previously submitted applications.
I am one quarter American Indian, you would never guess that by looking at me, but it is my prerogative to report my race as an American Indian on any application I submit.