I assume it's a 12. So, I'd buy the Gaugemate Golds which are made to stay in the chamber while the empty is extracted.
If you went from 12 to 20, it would be a continous stainless steel cylinder which should take a lot of pressure off the chamber. I believe the ones from 12 to 16 have a longitudinal split. Call them to make sure. They might have some data on how much pressure is taken off of the original chamber.
I have patterned 12 to 20 a few times compared to the 12 pattern, the 20 ga Gaugemate out of the 12 bore gave about a 1" larger pattern.
http://www.gaugemate.com/purchase/gaugemate-gold
