ReplaceBodyPartR15 dynamically replaces a R15/Rthro limb part in a Humanoid with a different part. The part is automatically scaled as normal. In the image below, a R15 avatar has had his right hand replaced with a slightly larger version (also pictured).
This function is useful for modifying characters during gameplay or building characters from a base rig. The related function
Humanoid/GetBodyPartR15|GetBodyPartR15 can come in handy when using this function.