Using the pressure release valve on the corny lid, you'll need to vent the headspace CO2 slowly over time to reduce foaming--the beer will still be carbed. Once you stop hearing gas escape, you can vent and turn the pressure release valve ring 90 degrees to keep it "open", or vented. Now you have no head space pressure trying to drive beer out the fluid out tube/post; i.e., if (atmospheric) pressure is the same on both sides of the beer, you get no flow.
Now switch out the fluid out post (which you currently have as a gas-in post) for the correct one (no gash in the post). Leave the dip tube in.
Close the pressure release valve and re-carb by turning on the CO2.