Trans flush with no convertor plug...

If it's got over 60,000 miles & never been changed, DON'T TOUCH IT! If it's had a few changes since new, you're OK. If it shifts alright, just drain the fluid in the pan, install a new filter, refill with fresh fluid. (ATF+4?) Unless you can afford the fluid, (It aint' cheap anymore) what 4speedragtop suggested.