Moving existing borders can be just one big headache to you. I've experienced it recently because I'm working in a map for a "Teutonic" campaign, and I decided to use original borders as far as possible. The 1200 euratlas map that you've posted come in useful to split Poland in two regions, 1)Great Poland comprising Great Poland and Mazovia, and 2)Lesser Poland the southern part. As Martok says "You may end up having to do what VikingHorde did with XL", or having to do what WesW did with MedMod: 1)Poland comprising Great and Lesser Poland, 2)Mazovia. If you have free regions I suggest Samogitia (northwest Lithuania), a good addition if you include the Lithuanians as a new faction.