The change occurred during the course of post-war treaty negotiations, after the matter of relocation of populations had already been settled. The original border proposal ran along the Nieman. During negotiations, it was decided to move it further west, closer to the Curzon line, without revisiting the question of relocating the ethnic Polish population.
Do you use some specific websites for your research?
Various and sundry, depending what Google searches in Polish turn up. You're not going to find a lot of this information in English.