Please give me some advice and assistance on this one.

Here is what I have:

1999 Sentra Automatic wtih 84K miles.  The tranmission fluid is at the
right level and the right color.

The engine was recently changed and a transmission problem began.

If I start out in Drive, it will not shift from 1st to 2nd until 22
MPH--this is way too late and the car jerks into 2nd.  The 2-3 shift is
perfect.

But, if I shift the car manually from 1st to 2nd, it shifts perfectly.

What could be wrong and how would I fix this?

Thank you in advance for any help you can give.