change the blood elf skin color to a color that somewhat resembles night elf skin, job done, with features however it might be buggy due to a difference in night elf and blood elf bone structures.
or you could do it the hard way, which is i believe the proper way, load in both models into a 3d modelling software such as maya, 3dsmax or even blender and change edit the model manually to have roughly the same shape as blood elf model, if you do it this way then all you have to do is import the model itself into game and switch it with default night elf model, its quite a lot of work but the final model will look a lot better than anything else you can come up with using MWS or any of teh other tools
