You can try baking into 16bpp tif files, and then only converting into 8 bit precision once you are 100% done making changes to the normal map. Realistically though, in engine, this will get compressed and look much worse. I also don't know if DDO accepts 16 bpp normals. what kind of changes to the normals are you making…
Toolbag will load a 16bit PSD I believe, just resave your tif as PSD. Though the majority of game engines will not take a 16bit normal map, its just a really poor use of resources memory wise, so best not to stress too much about minor bit depth issues (again, that should go away when you actually texture the asset). Once…