As Az said, the structure with PBR isn't COMPLETELY different, you won't have to remake your UVs or anything crazy like that.
With PBR, you need the following data in textures:
Ambient Occlusion(i believe this is technically optional, but helps a lot in ensuring the nooks and crannies on a surface are shaded right).
While you can pass in the Metal, Rough and AO in as separate grayscale images, because Az rigged up a compositor function, it's going to be easier to work with if you combine all 3 into a single image and just tell the PBR mat to use that. Again, as Az said, Red channel is smoothness, Green is AO and Blue is metal.
Once you have that set up, you just need a cubemap. Level-wide will work, though we're trying to wrap up an IBL(Image-based lighting)-style system that uses area probes to provide regional ambient lighting info. You'd place a probe in the map, scale it's area of influence(if the pixels all inside it, it's cubemap is supplied for the PBR calculations) and then you'd either manually set a cubemap, or let it bake one from the probe's point of view. Idea then being that you sprinkle them around your level so that the lighting on objects is properly consistent to their surroundings.
For making the various maps, and assembling the composite texture, we highly recommend you nab Allegorithmic's Substance tools which you can nab from here
. Their Substance Live bundle has 3 tools that are super useful for cranking out PBR materials and they use a rent-to-own system where you pay $20 a month until you pay off the license, and then it's yours forever. So it's a pretty good deal for such good tools. If not, just putting them into the right channels via any image editing software will work fine as well.
We'll also be fleshing out the process of making and converting images and materials over to PBR on the wiki to make all this easier to keep on top of