Got home a bit ago so was able to tackle this. The hardest part was getting the nuts off the plastic piece which those posts hold the rubber in tack. I don't know how merrcat did it without removing those nuts as the posts is what holds the rubber in place.
EDIT: looking at it again, I guess you don't have to remov the plastic piece and stretch the rubber around it - that's what Merrcat was saying. So just loosen the 3 munchkin screws to get the rubber in and you should be golden...
Anyway, here's what you need for anyone doing it. Remove nuts to get plastic off using a 11/32 wrench; if you can get a ratcheting wrench that'll help a lot. The hard part is the post screw moves with the nut, I have no idea how I got it loose, but eventually the pinball gods just threw me a bone and it budged. You'll probably need a 5/16 to tighten the post back down. The munchkin playfield is easy; just remove screws 5&6 in the screenshot and loosen, but not remove, 8. That gives you enough clearance to get the rubber in.
I replaced the rubber with a 2 1/2 white; seems to work just fine.
Here's the tools you need and screenshot from the manual: