It's a Character-type that sounds like an Effect trope
It is confusing:
'glam' is a commonly-accepted abbreviation for 'glamorous' in the sense of 'beautiful'. It's not used as an abbreviation for 'glamor' in the magical-sense that the trope is trying to use it.
"Shazam" is in reference to Billy Batson/Captain Marvel, who changed from a boy to a grown man when he activated his powers. Shazam has lots of properties, not just the one being referenced here; the change from child to adult is not the most memorable of them.
It's only tangentially related to the trope: It requires a jump from "Glam" to Glamour, as in "a magic spell" and requires a jump from "Of Shazam" to "What Billy Batson has to say to become Captain Marvel", then requires a third jump from those two to "A Character who has the power to take on an adult persona at will via magic or phlebotinum."
Reasons not to change it:
The trope refers to both "Glamor" and "Glamorous"; i.e. the magical transformation to an older, more glamorous self.