Traditional beech laminate trees (spring or rigid) can in theory be widened and narrowed but only a certain amount, and a handful of times only. The metal headplate can be weakened by too many adjustments, or trying to change it too much in one go, especially narrowing it. Some fitters believe that once a tree is in a saddle it shoudlnt' be messed with,
I adjust the trees in the brands I represent and and fit, but some trees are easier to adjust than others - our very flat tree is a bu**er to adjust! I won't touch anything more than 5 years old generally as that tiny bit of fatigue in the metal makes it vulnerable to the immense pressures placed on the tree in the vice.
I had mine widened when I first got YP and then narrowed after he had lost the weight needed. Saddler said much the same as above that he could only do it up or down a bit and wouldnt want to be doing it very often.