It's not that easy as you guys may think it is. First reason the battle models have a different, often higher count of bones. Another reason is that issues of a mesh might not appear in battle but in the field because of the now in real-time calculated light. Often this are wrong face normals and bad placed polygons which alter the appearance of a model drastically.
But to answer the question you need to use kimera and have to manually replace the bone pieces of a field model with the one of the battle model.