From what James has stated previously, the BP26 is an evolution of the BP25 but as an evolution and not a revolution, there has been a refinement in the BP26 but it is marginal. Essentially, the soundstage will be a little larger and more defined, the noise a touch lower and the dynamics a little better with the BP26.
How noticeable these differences are depends on the rest of your system. With a higher the quality and more importantly the transparency / revealing nature of the power amplifier and speakers - together with the source quality, the more noticeable the difference will be.