The most immediate effect would be for the Polish Crown to both take ownership of church land and control of the church itself. I honestly don't know what percentage of land the Polish church owned, but every little bit would increase the crown's income, and thus its power vs. the nobility and sejm.
Also, though I generally agree with the above posters that the "Protestant Work Ethic" is overblown, Protestantism did encourage common people to personally read and attempt to understand scripture. Since one has to be able to read in order to do this, Protestant countries tended to prioritize education more than Catholic ones, and Protestant churches often tried to give common people some basic form of education. Related, since the church will be under crown control, the crown will have to have some method of giving clerics the higher education needed for their jobs, which probably translates into greater support for Poland's universities. This in turn might help to give the bourgeoisie/lower nobility more opportunities for higher education, and make it easier for the crown to establish an educated civil service.
As previously mentioned, relations with Prussia are likely to be better. I don't know if this will result in a "Polish-Lithuanian-Prussian" commonwealth, but I think we'd see more Prussians (and other Protestant Germans) at court in Warsaw, resulting in both more German influence on Poland and the Prussians identifying more with the Polish state.
Taken together, I think the above three factors will most likely lead to a more centralized government, with the able to exert more influence vs. the Sejm, possibly turning Poland into a centralized, hereditary monarchy. 19th century Poland might look a lot like 19th century Prussia-a highly centralized state with a strong civil service and military, dominated by its conservative nobility.
As to how Ukrainians/Belarussians do, historically Eastern Orthodox and Protestants tended to have a fairly good relationship. Russia generally treated Protestant minorities within its borders much better than Catholic ones, and Protestants didn't see Eastern Orthodox as "the enemy", at least to the extent Catholics occupied that role. On the other hand, Luther and Calvin both encouraged missionary work to all peoples in their native language, so the state church is likely to produce Ukrainian and Belarussian translations of the Bible, and make at least some effort to convert Ukrainians and Belarussians. Since any resultant churches will be Ukrainian/Belarussian speaking, they could still become the centers of nationalist movements, and Protestant Ukrainians might have an influence remenescant of Greek Catholic Ukrainians IOTL.
In the foreign relations department, Poland will obviously be much closer to the other Protestant countries in Germany and Scandinavia. In particular, I can see a long-lasting Polish-Swedish alliance once Russia starts becoming powerful.