On Monday, January 27, 2020 at 4:45:54 PM UTC-8, jp wrote:
I've searched the FAA Legal Interpretation website for an answer but haven't found one I understand.

My question is:

Can any of the flight experiences that were used to qualify for a private pilot certificate be subsequently used in part or whole to satisfy the flight experience requirements for a commercial pilot certificate.

Or, must the flight experience requirements for a commercial pilot certificate occur after the applicant has acquired a private pilot certificate.

Thank you.

Yet another data point. I just found this 2011 FAA Legal Interpretation that may address this question.
This is a reply from the FAA Office of the Chief Counsel to a question from Daniel Murphy:

"The FAA recently issued an interpretation addressing a situation in which a pilot wanted to use the experience he obtained while training for a private pilot certificate to satisfy the night cross country experience required for a commercial pilot certificate. See Oct. 8,2010, Letter to Richard Theriault from Rebecca B. MacPherson, Assistant Chief Counsel for Regulations. The FAA's interpretation did not permit the pilot to use experience that he acquired while training for a private pilot certificate to satisfy the commercial pilot requirements. Id.
Because a student pilot who is training for a private pilot certificate is not expected to perform at commercial-pilot-level standards, the type oftraining contemplated by
§ 61. 129(a)(3)(iii) is not interchangeable with the kind required for a private pilot certificate. See id.; 60 Fed. Reg. at 41181-82. Accordingly, any cross-country training experienced by a student pilot would not be credited toward the requirements of § 61.129(a)(3)(iii)."

I'll keep looking for more info.