This is implied by the rolling description but I hope it's not true -- if so I've ruined a couple of cars at least

It seems to me it would be more logical that the refresher puts the frame back to maximum rigidity regardless of the roll cage status.
Given the way the game works, I think it doesn't matter which order you do them in. This is kind of hard to express in words, but I'll try.
First, let's assign an arbitrary unit of frame rigidity, I'll call it FR. A brand new car has an FR of, say, 80.
When you buy a used car, it has an FR of, say, 60.
When you buy a roll cage it increases whatever your FR is by 25% (another guess.) Thus a new car with a roll cage has an FR of 100, a used car with a roll cage and no refresher plan has an FR of 75.
Okay, so you have a used car with a roll cage (FR of 75). Under the theory that the order matters, this car is locked at a max FR of 75, never able to reach the peak of 100 otherwise possible.
HOWEVER, the rigidity refresher plan is STILL available after installing the roll cage mod to an "unfresh" car. If the car is locked at a max FR of 75, and is currently *AT* 75 FR, then the refresh would become unavailable because it is already maximized. Therefore, it is reasonable to conclude that the car is NOT locked at 75 FR, and can reach 100 FR by applying the rigidity refresher plan modificaiton.
I hope that makes sense.
WodahsEht