Can i switch X Y Z in a quaternion?

Question

i have a coordinate system where the Y axis is UP. I need to convert it to a coordinate system where Z is UP. I have the rotations stored in quaternions, so my question is : if

Answer 1

No, you cannot exchange y and z -- it will turn into a Left-Handed Coordinate system, if it was Right-Handed (and vice-versa).

You can, however, do the following substitution:

newX = oldZ
newY = oldX
newZ = oldY

I suspect that what you really want is a simple rotation about the x axis. If that's why you want to switch y and z, then you should instead apply a rotation of -90 degrees about the +x axis (assuming you have a Right-Handed coordinate system).