Rather than start that new thread, I'll just lazily post in this one. Archie kept the gap in his front teeth after becoming cute; and since I literally had nothing better to do, I found that in Archie #92 (May/June 1958), he has no gap, but in Archie's Christmas Stocking #5 (1958, no month), he has the gap on the cover headshot, and in some but not all of the interior panels.
Conclusion: er, sometime in 1958.
Looking at the title Archie Comics, the tooth gap is coming to an end in the 1955/56 period. The last issue that has the gap on the cover is Archie #75 July 1955):
The first issue that clearly does not have the gap is Archie #78 (Jan 1956):
Archie #81 (July 1956) shows the classic Archie title (with the Archie head) that would become standard for Archie books into the early '70s:
I would assume that the change to the non-gapped tooth Archie on the "mast head" of the flagship title showed a definite editorial decision to eliminate the earlier version. As kakapo points out, however, there were scattered appearances of the "old" Archie for at least a few years. I don't know if that was stockpiled art or if some artists were just slow to adopt the new look