Who on A can shift?

[GPC2]

We had a discussion last night about which members of A can shift. I was unable to find any directive stating which members can/cannot shift. The only stipulation seemed to be that any shifting must be followed by one second of no movement prior to a snap or motion.

The reason I'm asking this is because some veteran members were saying that once a player is in a three-point stance he can't shift. They also were saying that interior linemen couldn't shift. I know that the general assumption is that once a lineman puts his hand on the ground he can't lift it up, but I can't find anywhere in the rule book that states that.

Any assistance is appreciated.