Changing gears in Auto while driving?

I have an Automatic 88 Sierra S15(2.8Liter) pick up and it has overdrive. Is it okay for me to switch into overdrive while I'm driving on say the highway? I just got this truck so just wanted to check. It has a new trans and don't want to screw it up. Thanks

Changing gears from D3 to overdrive while driving will not hurt your tranny. It is also ok to leave it in OD for most city driving. Even alot of stop and go. Downshifting from OD to D3 is also ok. I've done this for years to save the brakes when decelerating from highway speeds. Never a tranny problem. 88 Chevy K2500 (98K), 97 GMC K2500 (54K), 95 Jeep Grand Cherokee (45K), 01 Pathfinder (46K), all good to go. Good luck with your GMC. I love mine. LtScud

Thanks. I have been shifting it like that for a couple days and it does it pretty smoothly so I figured I would be alright.