The rabbi spoke today during prayer services and made the following points (inter alia; and I've no citations for most of them):
- The time to cease saying "מוריד הגשם" and start saying "מוריד הטל" is the first day of פסח.
- We'd switch as soon as that time arrived, namely at מעריב; the reason given for not doing so is that many people are not in the synagogue for מעריב and won't know to switch.
- We'd switch at the next opportunity, at שחרית; the reason given for not doing so is that we want to announce, before the עמידה, that people should say "מוריד הטל", and can't do so during שחרית as it would constitute an interruption between גאולה and תפלה.
- So we switch at מוסף.
- רמ״א says not to say "מוריד הטל" at all. He also says not to announce "we stop saying 'מוריד הגשם'", as it is like praying for no rain. Thus, the community switches only after the שליח צבור does so in his repetition of the עמידה of מוסף, and there's never a pre-עמידה announcement. (This is מנהג נוסח אשכנז outside of Israel.)
It seems then that according to רמ״א the reason to switch at מוסף and not שחרית is nonexistent. So why not switch at שחרית?