It might be to see if it is more palatable and by improving feeding/ diet overall he may be better. Obviously it shouldn't be assumed that there is soy intolerance, and it is around this age you would diet trial these things to confirm or discard the intolerance issue.
Sometimes their tummies just seem better on one for no reason. Just think of all the normal babies on all the many normal formulas
Yes - it does take an adjustment period for a formula switch.