Perhaps Snape switched the horcrux locket?

[Auria]

No: HPFGUIDX 134230

Regarding the fake horcrux locket containing the note from R.A.B, I 
have a theory: Perhaps the real horcrux locket WAS retrieved by 
Dumbledore and Harry from the cave (as Harry mentions thinking the 
locket seemed smaller than the one he saw in the cave). But sometime 
between then and Dumbledore's death the real locket was switched with 
the fake one. This could have been done by Snape when he escaped with 
Draco before Harry caught up with them in Hogwart's grounds, because 
Dumbledore's body was lying below the tower. This could be part of the 
whole 'Snape is really a triple agent and is really on the good side' 
and maybe Snape has a plan to destroy the real locket.

Could this make sense? Anyone have any other theories on this??
Rita