Forum:Card set name change... confusion!

I'm sure this has been asked elsewhere but I can't seem to find it and am new to this site, but I've always wondered ever since it occurred, why was there a sudden change in the card set codes, I think thats what they're called, the little code beneath the card picture which usually goes something like XXX-EN001.

For sake of argument I'm gonna use Blue-Eyes as an example. In the older cards like the original starter deck the code was SDK-001, but then on another Blue-Eyes I have the code is LOB-E001, then finally I have a third with the code LOB-EN001.

Is there some sort of difference between the 3 cards. And am I still able to use all three in Advanced Format duels and in tournaments? Also as a side question on the same matter as I'm guessing ones with E or EN are older cards can they still be used in duels with new rules?

Thanks for any answers people can give, and sorry if this is just a noob question.