I've done the seafoam, before, and after, didn't notice a difference either time.....the hooplah behind doing it "before" the OCI is the off-chance that some of the seafoam leaks down into the oil.....BUT, have no fear, Seafoam is SAFE for motor oil use!
The whole deal with changing plugs afterwards is more hooplah....typically, what happens on older vehicles, that have extreme carbon build up, the seafoam will "known some loose" that messes up the spark plugs, or in the off chance an o2 sensor....as the seafoam burns up, and passes through the tail pipe....
So no, I wouldn't change out the plugs unless it is "that time again".......
1 thing in closing, add the 1.5 oz. per quart to the crank case first. Then add the recommended concentration to the gas tank (typically 1 oz. per gallon of fuel in the tank....), THEN let the vehicle warm up first, then throw the seafoam down the vac line of the warmed up vehicle.....it'll be like a triple clean, you got the seafoam in the engine cleaning up, you got it in the gas, cleaning the fuel rail, injectors, etc.....and then you'll have it going through the intake, to an extent "meeting in the middle" with the seafoam in the fuel.....for a good cleaning effect.