The force required will not vary with internal pressure. You already have the bench test. What more do you need?
If you apply a prescribed displacement on the piston to move it the distance necessary, if you use result force you will find out how much force was necessary to move it based on resistance from the pressure and friction.
How are you going to handle the pressure and its change when the opening process happens?
As I see this it is a balanced piston so no force is required to overcome gas pressure when moving the piston, only to overcome friction between O-rings and housing; this will vary with pressure differential across the O-rings.
I don't think any SW simulation will help you here, even if you know friction coefficients.
According to what you have modelled I suggest you will have great difficulty reseating the O-ring.
If your results are deforming piston or body then I agree that it is a long while since you used simulation and I question what you learned then.
If it was me: I would use a Non-linear 2D static study, put the O-ring in, add contacts as appropriate, use the perscribed displacement as Jared suggested, use a reasonably fine mesh, with friction. If the defromations of the o-ring are quite large you may need the NL properties to define it as a hyper elastic material but you could try it with linear properties and see where it gets you. You may have to set up the o-ring compressed state as the inital step first if you are trying to relax the o-ring and make an opening. I would use a constant step size of about 1/2 an element size or less. You may want to take a look at tutorial or training material before jumkping into this. Good luck as it will be a bit tricky to pull off but can probably be done with Sim Premium..