The PNP can do a bunch of things. It can cause te tranny warning light to blink, it can cause your car to go into limp home mode, it can cause rough clunks when shifting in and out of R and D, it can cause your car to be unstartable.

If you replaced your starter and alternator yourself, this job is EASY in comparison. The only thing is, you need an allignment tool to allign the PNP with the tranny. So if you are tight with the techs at your dealership, they'd probably lend it too you for an hour or so, which is all it takes. It's actually a really simple tool. It basically sets the PNP switch to either P or N (I can't remember), and then, all you have to do is shift to P or N inside the car, then tighten, and your done. Talk to your techs. Many of them do the change without the tool, so I'm sure they'll be able to give you some tips on how to do it without the tool.