Transmission slippage?

I recently had my tranny replaced with one from the salvage yard with 38k on it. For the past 2 months the car has been driving flawless until this week. After I start the car up and put it in drive it revs up to 4-5k rpms and shifts hard.Once the car is warmed up every gear is smooth sailing. Ive avioded this in the past by waiting for the car to warm up to about 120 degrees but how can I fix this..Is it possible that the fluid is old and just needs to be changed?.. or the screens are clogged? Im just afraid to persue anything without any research