Jose;
I have no idea what the actual impact on pressure would be. It is however generally considered that very fine shot in comparsion with very large shot, the fine will give a higher pressure level than will the coarse. This is brought on by there being much greater bore contact, thus more bore friction, with the fine shot. In loading manuals buckshot loads are virtually always listed seperately from regular shot loads. I cannot positively state you would have any problem with the switch, but I for one would not personally recommend it until you found someone really in the know as to what the results might be.
