Changing Secret Keepers (Theory)

[Hagrid]

No: HPFGUIDX 81069

We have seen Memory Charms that can be given with few people around, 
if any, realising the mind has been altered. I refer to Marietta 
Edgecombe. Her memory was modified by a timely Obliviate spell from 
Shacklebolt while Fudge and other MoM aurors were in the room. (OotP)

Minor alterations may not need the flourish of wands a full Memory 
Charm like Lockhart prefered (CoS). 

If so, did someone hit Sirius with a memory charm? -A suggestion that 
influenced him to swap secret keepers? The way the Potters' died 
would be impossible without switching secret keepers, and the choice 
of Wormtail was just TOO convenient for many mystery writers.

This may appear in book 6 or 7.

The memory charm would need to be done by someone close to Padfoot. 
Very likely, the same spy that kept giving OotP secrets to Voldemort. 

Pettigrew was called "talentless" by Sirius (POA) and "Hero-worshiped 
Black and Potter. Never quite in their league, talent-wise. I was 
often rather sharp with him...Stupid boy...Foolish boy...He was 
always hopeless at dueling..." by Minerva McGonagall.

The Death-Eaters would not accept Wormtail easily since he lacked 
ability, so I suppose there was a 2nd spy within OotP - Again, that 
gets me suspicious about Snape. .... aussie