why doesn't my car shift into overdrive when cold? on 1994 Toyota Corolla

this problem only occurs when car is cold. when it is warmed up it works fine.

It's not supposed to go into overdrive until the engine reaches a certain temp. to improve emissions controls. Is it taking longer for the engine to warm up? Maybe thermostat is opening to soon or is stuck partially open. If it concerns you , get it checked and verified before randomly spending money on parts.