Indeed, but before it has been conclusively determined I must consult and confirm or interpret the source for this:
Nai
When we assume, that the Force did shift towards the Dark Side, than Plagueis delivers the basic idea himself:"But had Sith like Naga Sadow and Exar Kun genuinely been more powerful, or had they benefited from the fact that the dark side had been more prominent in those bygone eras?" - Darth Plagueis, iBook, Chapter 5, p.65
And after the shift occured, the first thing Plagueis does is demonstrating his - apparently new - ability to bring beings back from the other side of the wall of death unaided.