Remember to account for not only the width of the tire, but also the location of the widest part. If you run a wider 27.5 to make up the same diameter as a 29, then the widest part of the tire will be not only wider than on the 29er but also closer to the axle. Depending on how your frame is shaped this may or may not be a problem.

If you run the same width tire and just a smaller diameter then the bike handling will change. It could be bad or it could be just different. Obviously the BB gets lower but also the trail shrinks as you put on a smaller diameter tire. This would generally make the bike a bit less stable. How much depends on the rest of the bikes geometry. You may like it or you may not. Considering road bikes are usually made with much more aggressive front end geometry than MTB's, it is not likely to be unrideable but may not be great for a MTB trail. Give it an easy test ride before diving into anything aggressive.