Hi @James R. Coppernoll ,
Welcome to Q&A forum!
Per my research, I also did not find any updates about the downgrade permissions of Office Standard 2021. But I can confirm the following information:
Downgrade rights in Commercial Licensing programs give you the right to downgrade to any prior version of the same product. Therefore, you may downgrade Microsoft Office Professional Plus 2019 to the Microsoft Office Professional Plus 2016 or Office Professional Plus 2013.
And I would suggest that you contact Microsoft support directly to inquire about this issue.
Any updates or questions, please let me know.
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