Yekke2, aside from the Igros Moshe mentioned, there are other sources as well. In one of the Shabbos App threads, the topic of zilzul Shabbos/uvdin d’chol came up, and I brought o couple, starting here: http://www.theyeshivaworld.com/coffeeroom/topic/the-shabbos-app-controversy/page/2#post-543925 .
Interestingly, earlier in that thread, Sam mentioned the switch (I think he meant the one discussed here). I responded that R’ Neuwirth had retracted (it turns out that it was a clarification; he never meant it l’chatchilah).