I think its important to explain why i dont agree with a CB playing safety - particularly strong safety:-
The best way to stop offenses is by getting interceptions. If a team has good LB's then the SS becomes more effective by picking passes. One of the more 'unsim' developments in Madden is the superhuman TE's and we have all seen them get big gains by running downfield towards the SS cover area. This is not coincidence:-
The majority of decent SS's (excluding the top couple of guys) have a catch rating in the 60's, which is on average 10 points down on CB's. Also, CB's are generally 7-10 points faster than SS's. The only area a SS seems to have the upper hand over a CB is run stopping - but this trade off doesn't balance out as an average CB will make far more picks playing as a SS than a true SS and therefore becomes more effective, plus the added point that some CB's are also good tacklers.
If this is ok - we could argue that HB's and WR's can be inter-changeable too - and this would also be unsim in my opinion.