When you have a glow discharge, you don't have an arc, which means that your plasma's conductivity is rather low.
That being said and assuming that the voltage is high enough in any case to sustain a discharge, the following will happen:
1. p decreases: The voltage should decrease at first because free electrons will gain higher kinetic energy between two ionization collisions. However, at some point, you will need a stronger electric field to produce enough electrons to keep the current constant --> the voltage raises again from this point on.
2. p increases: the voltage will increase because you get much more collisions between electrons and neutrals and you need enough free electrons to sustain your current (but few enough to avoid arcing). These collisions dissipate energy from the electric current, which you have to compensate for. This can either be done by increasing the current, and/or the voltage. Since your current is fixed, the voltage has to increase.
