Can I Interchange Wheel Sizes On One Frame? So, I'm looking to see if I can build a bike and on the one hand I want to use 26" wheels, but at the same time, I want to use 27.5" wheels for another purpose. Is it possible to have a frame with a certain build (maybe a slopestyle or 27.5" enduro/downhill frame) that can use both 26" and 27.5" wheel sets? (I don't mean a combination of one 26" and one 27.5" wheels together on the bike at the same time, I mean one separate set of 26" wheels fitting on the bike and a set of 27.5" wheels that can also fit.)

Thanks very much!
If you use disc brakes with same size rotors, you can put 26" wheels on a 27.5" bike. 

In fact, you can put smaller wheels on any any frame, so you could put 24" wheels onto a 29er 

You need to watch for the a few things - geometry is going to be affected, and you may get pedal strike as the bottom bracket is lowered. In the case of 26" and 27.5", the difference is small - 26" is ISO 559, 650B/27.5 is ISO 584 - diameter difference is 25mm, or 12mm radius(BB height).  

You will need to watch the axles style and length for the wheels - an array of mostly incompatible new standards have come out in the last few years so you will need to ensure the wheel sets match the frame/fork