1993 Toyota Corolla Does not change gears when its cold

I own a 1993 Toyota Corolla DX 3 speed automatic transmission. My problem is that when the car is cold, it does not change into a higher gear. I have to wait for the car to warm up, before it changes gears. Wait time is about a minute or two. Once it warms up, it starts changing gears without trouble. I looked in a lot of places for an answer to my question, but I couldn't find any information about it. We changed the transmission oil and filter, and it still does the same thing. Its not losing any oil. Hopefully you can help me out, because this is starting to drive me crazy. Thank you for your time.