1998 Saturn SL2 replace tranny...now has problem

I had just replace my tranny with an earlier tranny..it runs fine for about a mile the it starts to slip..is it because of an older tranny,,that my pcm is not working with it? I've also replaced the valve body and still the same..did I not replaced it right... I never pulled the T-converter out just slip the new tranny in place...do I need to pull it out and put it back on before the new tranny slips in there...thx