Could they relocate from the German area to the Russian area or from Russian to German?
I read that in the 1880s the Prussian government deported all Poles that did not have German citizenship to "Congress Poland", in order to reduce the number of Poles in Westpreußen (Royal Prussia or Pomorze), Provinz Posen (Greater Poland) and Oberschlesien (Upper Silesia). This was pretty harsh because many Poles lived for decades in the above mentioned provinces, they just never cared about getting the German citizenship.
After this procedure crossing the border between the German and the Russian Empire wasn't so easy at all without further ado. As a German citizen you needed a Passport that was rather expensive and wasn't given to politically "unreliable" people, likes patriotic Poles. Relocating from "Russian Poland" to the German Empire was almost impossible for people with Polish ethnicity. Ethnic Germans from Russia in contrast could relocate whenever they wanted.
Of course the frontier was not fortified, so it might have been possible to cross the border by foot illegally and make a visit to the neighbouring village on the other side of the border line. Perhaps some of the German Border Patrol men even tolerated that, but it surely was better not to get caught.