Rogue Cronus tap change question

Just got a Cronus Magnum off Audiogon and I'm trying to change the 4 ohm tap over to the 8 ohm tap. I just can't seem to get any type of wrench on that nut to make the switch!! Any advice from current owners would be appreciated.
Call Rogue directly. The best customer service I've ever experienced. Congratulations on your purchase. The Cronus Magnum is a flat out amazing bargain.
I used a 12mm wrench and performed the procedure at the kitchen table. It was really quite easy once it was on the table and off the rack.
Yes it's a 12mm needed. I changed mine to the 4 for my Triton One. Tasty combo btw.