when editpos is used to turn a character by updating YAW what memory vars get afftected, is there a set algorithm you use / create to get the facing right?
All 9 elements of the 3x3 part of the matrix that represents orientation are potentially affected by a change in yaw. The middle 3 (second column) will probably remain constant. There is an algorithm, but if you understand the math behind it, you're a lot less likely to run into issues that you can't solve. I'd highly suggest looking up transformation matricies in a 3D graphics concept. Use of them is quite common in game programming, so there is a ton of material on the subject that can help.
also what is meant by a Ypr order of rotation?
A matrix is an unambiguous way to represent orientation. Disregarding floating point error, there is only one matrix that corresponds to a particular orientation and vice versa.
Euler angles on the other hand, are ambiguous because not only do they "wrap around" when outside the range of 0-2*pi (or 0-360 degrees), there's also the question of what order to apply the rotations in. Do you pitch the object up 30 degrees, then yaw it (along the new axis) 50, or do you yaw 50 first and then pitch up? Those two operations do not result in the same final orientation.
When processing PYR from something like a demorecord and composing it into a matrix, even though they are in the file in Pitch-Yaw-Roll order, COH applies Yaw first, then Pitch, then Roll.