Lately my transmission has been missing gears. I would accelerate and it wouldn't shift and my rpms would go through the roof.

Im right at a 30,000 mile interval from when I originally bought the car and thought about getting the fluids flushed but it occurred to me that I have no idea if its ever been changed or not.

Question is.. Without knowing the previous maintenance should I risk changing the fluid at 158xxx miles?

 

Trans flush at 158 is not the best idea.
ALso If you change the trans fluid, try dextron 6

 

check the fluid level and condition first. Does it smell burnt or is it brown in colour?

What gear is it stuck in? Will it upshift when in the manual gear shift position...m2, m3?

Any dtc present? Throttle position and Vehicle speed are the 2 biggest inputs for shift timing...does the speedo work?

60 000miles is the service interval for trans fluid service...hopefully you don't have crud in the valve body causing a stuck shift valve.