I had the same issue with a PSA dissipator upper I just received. I gave a two star review on that item.
Mine was a pinned FSB just like yours, however I was able to change to a phosphate FSB with some machine work. If you don't know what you're doing you can mess up the switch big time.
Since I have already drilled and reamed the taper holes there was no way to send my upper back.
Between the ridiculous looking FSB and the large gouge on my barrel near the flash hider, I am not happy. I also tried to change to Magpul MOE rifle length handguards and that was impossible. The distance between the delta ring and handguard cap was to narrow. The list goes on!
I should have sent it back, at $209.00 I thought I would practice my machining.
I've dealt with PSA for many years and this really surprised me, it's embarrassing.