I am not sure if Howard wants to change his opening theme song or not because I do not think I have heard him say that but I might have missed it, but if it is true the reasons would be quite simple to me:
1. The opening theme song says mother fucker and he feels he has to evolve and clean up his image
2. The further pussification of Howard by Beth. I am guessing she has suggested to him that he needs to change that song because of the mother fuckers in it.