I believe they were phased in over time but the timing is unknown to me. I looked at my old Buck catalogs and found that in 1983, no 100 series fixed blade sheaths were pictured. Then, starting in 1984 and continuing through 1988 (I did not look at 1989 or later catalogs), there is a picture of both the snoopy style and flap-over type sheaths with the 100 series knives. It is my belief (no clear-cut proof) that model 124s first received the snoopy style sheaths. I do not know just when the last flap-over type sheaths were offered but my collection has post-1986 knives with both styles of sheath and there does not seem to be a definite cut-off date, even when considering only one model. In other words, I have 119s with both types of sheath and they are mixed out of chronological order as regards to the year marks. The same holds true for other 100 series fixed blade models from that time frame.
I am sorry I cannot give you a more definitive answer. Because my collection is rather large, one would hope for a trend to be apparent, but I had no such luck!
By the way, "snoopy" type sheaths are those that snap around the side of the handle and leave the pommel exposed at the top. They are so named because, if one flattens out the sheath, the outline resembles the head of the famous cartoon beagle.