I can nail down some dates to pay attention to.
Bouchard wore a fitted mask (the beautifully painted one) in the Flames' game on October 13, 1977 in Boston. He was apparently wearing a different mask on October 26, 1977 against Washington, since there's this cool photo in the Des Moines Register:
I wonder if he was trying it out, which might (in conjunction with the photo) explain why he switched to a cage. My guess is that he switched to a cage in his return on November 2 (at Montreal), although I can't find a photo of him doing so until November 6. From then on, he's wearing a cage in every Atlanta photo I can find (through 1979-80). Some red and some white. The white appear to be Coopers, but the red all appear to be JOFAs (except that the logos aren't clear in any of them, save one that I note below).
EDIT: The Philadelphia Daily News has a photo of Bouchard - in a red cage that seems to say Cooper on the front - in the November 2, 1977 game against Montreal. No evidence of yellow stripe or logo, of course.
Bouchard was wearing a JOFA in that January 14, 1980 photo that I posted above (against Montreal).