I have a 2007 STUMPJAMPER front 26er with a 100mm RS fork. Can I use 27.5 wheels without modifying the frame or fork?
None of the frames I have modified so far were like this one, so I can't give you a definite answer, but I don't think the fork will work. The best thing would be to try it with a 27.5 wheel to see how much space there is between the tire and the fork crown. I would recommend doing the same for the rear wheel.
If you don't have a 27.5 wheel available to do these tests, there is a very empirical method you can use: measure the space between the dropouts and the 26 tire, both front and rear. If the space is greater than 2 cm, it is likely that 27.5 wheels can be fitted without any problems; if not, then obviously not.
However, this method is not 100% reliable, as it is important to remember that a lot also depends on the type of tire you choose to fit.