Transmission help!!
 

I have an automatic transmission in an 88 7m-ge. The trans seems to be shifting fine at low rpms. when i step the pedal to the floor and try to launch the car it slips out of gear. Im hoping i don't need a transmission rebuild. I checked the fluid level and it is ok. I imagine the trans filter has never been changed in the cars entire lifetime. Im afraid to change the trans filter because someone told me, if the trans is going out and i change the filter and fluid, then the trans will definitely go out! The strange thing is, is the car seems to be shifting fine if i drive normally and not race it around!