Herceptin and Tamoxifen are different drugs with entirely different actions. One does not replace the other in treatment. Tamoxifen blocks Estrogen and Herceptin deals with the over production of a protein. They are often given together. As the previous poster stated; if your tumor was Her2 positive you should definitely follow the Herceptin recommendation. Regards ...
It is my understanding that Herceptin is given for tumors that were Her2 positive. If indeed your tumor was Her2 positive, then you should take the Herceptin. It has been shown to prevent a recurrence in many cases. I don't know the statistics, but I'm sure you can find them on the web. You need to discuss this with your oncologist, since he/she is most familiar with your particular case.
Good luck with your treatment!