Can I Interchange Wheel Sizes On One Frame?
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