If you get an Operatic Belcher in a BG and then later pick up the Belcher Portrait greater trinket, the +15/+15 effect when losing venomous does not apply to the existing belcher, only the new one(s) you get.
If you get an Operatic Belcher in a BG and then later pick up the Belcher Portrait greater trinket, the +15/+15 effect when losing venomous does not apply to the existing belcher, only the new one(s) you get.