Mostly posted in case I ever have to do this again, but also to hopefully work this post up to the top of google to help someone else out as the answer to this was buried in tons of spam sites with links to toner, drum, etc replacement sites and extremely little help on actually zeroing the drum copy counter on a Xerox Workcentre (xd125f in my case but it seems xd120f is also a common one people were trying to get help with and seems to work for it as well from what I read). The printer gives a j2 error on the screen when it hits it’s max copies and just changing the drum isn’t enough to reset it.
First, you need to enter diagnostics mode:
With the printer off to start, you need to turn it on and within the next 4 or 5 seconds, push Clear, Exposure Mode, Clear, Exposure mode. On the xd125f that I did this on, Clear is to the far right and Exposure is the far left button.
The display will go blank but the printer will still be running. Select 24 copies and press start and the screen will go blank again. Select 7 copies and press start again, this will reset and show 000 on the screen.
Turn off the printer, count to 5 and turn it back on to what should be a functioning printer again.
Not an overly hard process, once you know the combination of keys and the right order. Of course, finding that combination made me want to smash the printer into little bits aka Office Space (As a site note, there are tons of people copying the Office Space printer smashing scene, in fact, so many of them that I couldn’t easily find the original scene to link here like I had wanted to)