There are desinated sides on a water heater because the cold side has a dip tube (approx 40" ±) that brings cold water to the bottom of the tank for better heat distribution. If the dip tube deterates as some manufactured in 1994-1996 by perfection corporation the cold will mix with the hot and the heater will be less effective.
I am a former lisenced service plumber with over a decade of experiance. Frankly, I would be cautious of a plumber who does not have this basic knowledge.
Tony