I'm trying to estimate the distance with stereo system of far objects. I have rectified the images acquired after calibration process and I have built the disparity map.
Is it now possible to change the baseline (and the maximum depth achievable) through changing the stereo calibration parameters; for example vector T or image center Cx contained in camera matrix?
Given that your hardware is unchanged, you cannot change any parameters related to that. So the baseline and camera matrices are fixed. Baseline, lens parameters and pixel size together determine the maximal depth resolution you can achieve.
You might be able to slightly improve your result through superresolution techniques or some other trick, but this will not dramatically improve your results. Especially for faraway objects.