I was just doing some reading on my Mossberg 590, and I stumbled upon a part that slips into the barrel (receiver end) that will convert the current weapon from 12 ga to 20 ga, and a variety of other sizes. From what I understood the part (which looks like a choke tube) is the only thing that needs to be changed.
I'll see if I can find a link to what I'm talking about.
HOWEVER, I am not an expert on these matters and I would consult either a gunsmith and/or your manufacturer before trying anything.
The short answer is you cannot change the gauge of your shotgun, either the Stevens or the Mossberg. They do make tubes that can slip into a break action gun, like the Stevens, but you could buy another gun for less than the cost of the tubes.
What could have happened... did.