Does it hurt to shift an automatic transmission?

I own a very old car (1988 ford taurus) and it has an automatic transmission. I'm a real freak about merging onto highways, so I usually shift it from the overdrive gear into the drive to downshift it (Pushing the pedal to the floor doesn't always downshift it) and try to get up to speed as quickly as possible. So the question is, would this have cause my need to rebuild the transmission after 67,000 miles?