I got an asset which is quite high poly (50k), it has all kind of textures including normal map for the fine details. I want ot retopo it to about 5k.
Is there a way to bake everything to the new low poly (different uv map ofc).
Yes, you can.
Any idea what do I do wrong? first I wanted just the base color…something happened, but the new texture is too dark.
That’s because you baked Diffuse (which means lighting).
ok, I’ll try. thanks
If you just want to transfer the map as-is instead of baking in the scene lighting, in your HP material, assign that map to an emission shader and bake the emission channel. Also remember to turn off color management.
The color map worked well this time… the normal however is not good… first the terrible seam and also it seems a bit weaker than the original. (this is btw just a test setup a quick autoretopo and v map, I will make it better once I know that the bake can give good result)
Blender is kind of finicky, I wonder what adjustment changed the normal bake? 
Or did you transfer the normal map?
I used to do all my bakings in substance painter, but there I don’t think i have options to assign texture to the high poly.
Now I tried to play with inverting channels before or after the bake, but the seam remained… no idea how to fix it
edit:
Actually the normal map works fine in substance painter :)… so i guess it’s just blender display… and since I want this in ue4 anyway it’s not a problem for me
Keep in mind that the normal in blender is inverted (due to openGL vs directx), you have to set the y axe to negative in the bake properties (-y).
I tried that, but it didn’t help
I’m confused now what worked in substance, the normal from blender or the normal from substance bake?
I exported the normal map that I baked in blender and looks bad there, but in substance it’s ok.
You didn’t set the normal map image node’s color space to non-color. Hence the seams.
That’s rigth, thanks!