Bonedaddy is a script for 3ds Max, developed by Chuggnut, that handles the weights of a skin-modified object. When you change the mesh of a skinned object, any manually set up vert weights get lost. Bonedaddy saves those weights out and lets you put them back, so the work you did on your skinning isn't gone after a mesh edit.
The script gives you two ways to restore weights: by index or by position. Both aim to return the weighting you had before the change. The developer cautions that putting weights back by position can take a long time if your object has a lot of faces, which is worth keeping in mind on high-density meshes.
The version listed here is 2.3.5, and it's available through Chuggnut on scriptspot. For the download, along with any usage notes, the developer's page at chuggnut.com is the place to go.