common and relatively safe drugs reduce breast cancer mortality

I think I read somewhere that the ER+ tumors start out as PR+ and then when it turns into PR- it is a bad prognostic sign. Can't recall where I read it... does anybody know anything about it?

(I forgot to add the article (which I can't find right now) which i read yesterday about how Tamoxifen makes the tumor cells both less ER+ and especially PR+ so this is relevant for everybody.
Is tamoxifen really just working like chemo ? causing resistance and survival of cells that are more aggressive?

Im not looking for a 5 year or even 10 year survival with a fast KILL in the end because of accumulating resistant cancer cells. I still don't dare to stop taking tamoxifen but Im starting to feel it might just backfire in the end??