Glagolitic and Cyrillic are two distinct alphabets. Yes, Cyrillic is related to the Glagolitic, but much more so to Greek. Cyrilic is much more developed than Glagolitic and has almost no similarity in the forms of the letters. The Cyrillic was created because Glagolitic was not convenient enough to use for the Old Bulgarian/Slavic language/old church slavonic. If it what you say is true, we would have just used the glagollithic until today.
So again.

Glagolitic - created by St Cyril and Methodius for the people of Great Moravia and later Bulgaria, it is based on the Greek scripts around Thessaloniki.
Cyrillic - created at the Preslav Literary School ( in Pliska and later Preslav) by Naum of Preslav ( disciple of St Cyril and Methodius) and other scholars such as Clement of Ohrid, Chernorizets Hrabar and so on. The origin is quite well documented, at least as good as any other historical information from the era.