From when did they lower the fiddle two half notes to play more "easily" together with a C-accordion ?

It's always been my understanding that cross tuning was accepted in early Cajun music even before accordions showed up, so I'd imagine pretty early on once the C accordion came into the picture.

John

Country/hillbilly and bluegrass influences may have come into it too, a lot of that stuffs in G and C.

This is very interesting. D would be the natural key to go with the fiddle. The forces that moved the standard to C are probably complex, but have to do with changes in music and also a need to lower the singing voice. Fiddle tuning followed along with it. The fact that much popular music from "english" traditions were in C and G certainly had an influence.
I do know that when playing by myself, I tend to prefer playing my D box, actually playing in the key of A most of the time.

JOhn, I prefer many songs, especially 2 steps, in D. When I play in C, they sound a little "bassy" or "gurgley", if you know what I mean.

Gurgley...that works for a description. The D box does seem cleaner etc. Of course, I like irish tunes which are most commonly in D or A anyway. At times, I will close the low reeds on my C box. In fact, I think the old players messed with the reed combinations more than we realize. Listen to the "Creole Belles" and you will hear several different accordion "sounds", from low and growling to clear and high. If you have a fairly wet tuned box, closing one of the middle reeds works well also.

yall got me convinced that my next box will be a D.

HI I have a couple of HA-114's.. C and D box.

Biggest difference between the two is the D box uses much less Air.

Other than that every pretty much the same.. Must be due to the smaller reeds.. I have read the G box is a real air hog. so will never go there.

I also have a 30's old style box in F.. Terrible key.. Very high pitch.

Johnnie

Don't throw away, sell or get rid of your C-box, because you're pretty much unhanded on most jam-sessions