Each time the belts are changed in tightness, the steps per mm need to be calibrated if you are looking for accurate cuts.
Here is a video on how to do so:
Here’s another by @RobertA_Rieke
As a side note, if you move your gantry by hand then it will move out of square. Be sure that you only move the gantry with jog controls or if you have to move it by hand then re-square after each time.