Not shifting

We sent van to have transmission rebuilt. Mechanic said we needed a new computer and that it would need to be re-programmed to our van then it will work fine. We put in an ECM, but have been told contradicting responses. One it does not need to be re-programmed. One says it does. Several say regardless it is not the problem (its not the computer). The mechanic will not get back to us. If he did the job right, what could the problem be? It is like it is stuck in third gear, no first or second or in limp mode.