Hm, the spellcard "signs" are different on the early Reimu, Rumia and Cirno ones. Reimu's Fantasy Heaven is "Dream Charm" instead of "Dream Seal" and it's written in four characters instead of 2. It'd be more like "Charm of Dreams," I guess. And in picture 7 you can see "Moonlight Ray" at the top, but instead of "Moon Sign" it's "Moon Charm" or "Charm of the Moon" in the same 4-character format. But in the Cirno one, it's back to the standard two-character format, although Diamond Blizzard is now "Snow Charm." The "charm" is probably used in the sense of "card" although it's like the cards Reimu fires.
I guess the only significance is ZUN trying to making the format more compact (4 -> 2) and then less material (Charm -> Sign).
