If you boot into maintenance mode you can reset the printer and it will boot up and accept the PK.
How to change from matte black to photo black without wasting money and ink
How to change the photo/matte ink in the Epson 7600 and 9600 printers without using the costly Epson draining procedure:
1 - Make sure your printer is switched OFF!
2 - Open the ink cartridge lever in the ink cartridge bay
3 - Replace the photo black cartridge with the matte black cartridge (or vice versa)
4 - Leave the ink lever open !
5 -Using the printer control panel go into Maintenance Mode 2 by switching power ON while pushing the 'Paper source', 'Cut/Eject', and 'Paper Feed down' buttons.
6 - Press the 'Paper Feed down' button until you see "SERVICE CONFIG" on the display - press the
'arrow right' button. It then shows 'XXD' in the lower part of the display.
SERVICE CONFIG - XXD
7 - Press the 'Paper Feed down' button until you see
"NPD" - press 'arrow right' button.
7a - A value of '1' appears in the display.
8 - There are 3 possible value settings for NPD:
0 - no ink check
1 - pigmented ink (there should be s star "*" at this setting)
2 - dye inks
9 - Change NPD setting to '0' (no ink check) by pressing the 'Paper Feed up' button and press
"Enter" - a star should now show next to the '0' value
10 -Switch printer off
11 - Switch printer on
12 - Close the ink lever in the ink cartridge bay
13 - Printer and driver shows the matte black ink is installed (or photo black ink)