Can I simply switch from oil pressure to gas pressure shock absorbers?

Technically, this is not a problem at all. After converting to BILSTEIN B4 shock absorbers, for example, you will notice improved, safer driving stability and road holding in every production vehicle.
Oil shock absorbers are only used to a small extent in new vehicles. During longer car journeys, there is a loss of damping force (up to 35%!), which only disappears again after a break. A gas shock absorber does not have this problem because the nitrogen gas in the shock absorber prevents the oil from foaming.
So when a damper replacement is due, gas pressure is always the technically better choice, no matter how old the car is. If there is a way to optimize driving safety, especially in older vehicles, it should be used.