Daas Yochid,
I read your post carefully and found that I agree with almost all of it.
Therefore, I would like to elaborate on, and even rephrase what I wrote.
I did not intend to claim that a community loyal to halachah “made a change”. Quite the contrary. If there was a “change made”, why do we not find one German posek who cites this change and explains it? We are met with German halachic silence, that’s why we are having this wonderfully civil discussion.
In any case, changes in minhag, even takanot, evolve over time, and are accepted in varying degrees in various locales over time. Even a cut and dried takanah like Rabeinu Gershom’s outlawing bigamy, was not accepted immediately, but was gradually accepted in most places over a few generations or more.In many areas it was only partially accepted. It takes time. For the record, it is highly likely that Rabeinu Gershom’s takanot were mostly not decreed by him, but that’s a separate topic.
In essence, your comments strengthen my theory. This gradual change over time (50 years? 100 years?) was so gradual that it never happened by psak or decree or halachic decision. Otherwise, we would have known when, how and by whom. There would have been some sort of halachic discourse. But there is none.
It could very well be that the six hour European minhag was not iron clad ,the fact that the Rama thinks it’s a good idea testifies that it was not uniformly practiced in his time. If so, the Mizmor L’Dovid’s referral to meal intervals (If I remember correctly, the Pri Chadash says sholosh oh arba sha’ot?)would fit my theory perfectly. Same for the Chayai Adam’s wonderfully ambiguous “aizeh sha’os)
So I’ll rephrase and I hope this is an improvement. There was as yet no clear cut axiomatic number of hours in Europe, with many keeping six and many less than six. In Germany the change in meal times did not allow for more than three, and so eventually, over several generations, the German minhag was set to three, while the rest of Europe gravitated towards the Rama’s preference for six. The Germans, not feeling bound by the Rama, and never considering six because that just didn’t work into their lifestyle, settled into three-ish. Eventually, people defined the minhag by the specific time period of three hours, but that was for the convenience of setting a defined time interval. Of course, according to my theory, it just as well could have developed into four, or two, or two and a half. It just didn’t turn out that way, or as I say it, it happened by sociological necessity and convenience. And that is why it slid into general practice under the radar of the written psak.
In conclusion, this is my theory and everyone is welcome to accept it or reject it. It will definitely remain a halachic curiosity.