Idle up is more of a function related to the head speed throttle curve.

Collective pitch relative to fixed pitch is more of a description of the pitch of the rotor blades. The two perameters that control lift from a rotor is the rpm of the rotor and the pitch of its' blades. So one can keep the pitch constant (e.g. 6 or 7 degrees) and vary the speed of the rotor to increase or decrease the lift required to climb into a hover, maintain a hover or climb from the hover. This is what is considered Fixed Pitch.

With a Collective pitch system, the rotor speed is set to stay constant, while the collective pitch is increased or decreased to climb (more lift), hover constant lift equal to the weight of the heli) or to descend, (generate less lift then the weight of the heli).

One can easily set a CP heli to fly like an FP heli by setting a variable throttle curve in the radio (so the throttle stick will increase and decrease the power to the rotor) and by setting the collective pitch curve to be a cons tant at say 7 degrees at all stick settings.