Can I keep using my STEM OPT EAD even though I get my c9 EAD?

I do not believe that it makes much difference which EAD you will be using. You are authorized to work under your STEM EAD and that is not revoked by your applying for adjustment of status and obtaining employment authorization (C )(9) on the basis of that. However, please note that the STEM EAD is tied to the field of your degree while a (C )(9) EAD is open market and can be used for any employment. Due to the limitations of the Lawyers.com Forums, Alan Lee & Arthur Lee, Esqs.’ (the "Firm") participation in responding to questions posted herein does not constitute legal advice, nor legal representation of the person or entity posting a question. No Attorney/Client relationship is or shall be construed to be created hereby. The information provided herein by the Firm is general, and requires that the poster obtain specific legal advice from an attorney. The poster shall not rely upon the information provided herein as legal advice nor as the basis for making any decisions of legal consequence.