In order to switch between MK and PK carts/inks in your R2880, the chip on the different black cartridge that you install must read full or nearly full in order for your printer to accept it and do a black ink mode change. If you have been using MK ink, then install a PK cart that is previously used, and the chip reads less than about 1/2 full- the printer will determine the cart doesn't contain enough ink to preform the black ink change, and the printer won't accept the cartridge.
If your PK cart chip is reading too low for the printer to accept it and do a black ink change, then you can manually reset the chip to read full again using a chip resetter. The chip resetter you'd need for a R2880 is:http://shopping.netsuite.com/s.nl/c.362672/it.A/id.5752/.f?sc=11&category=27717
I hope this helps. Please let me know if you have further questions or there's anything else I can help you with.
Best regards~ Dana