70/73 swinger rear bumper difference

73 lights are the same as 71/72 (4 light assembly's vs the 70 2 light assembly). How different is the bumper itself as they physically look the same except for the lights. I can locate the assembly's but need to know if it can be done since I have gotten different information, I know that 71-73 are the same and I can't see the mopar would have made that much of a change in one year, has anybody done this, I believe the scamp model used this bumper past 1970.