Sometimes, after carefully assigning weights manually, you discover that you need to make a substantial change to the enveloped object, such as adding points. Luckily, you do not need to redo all your weighting — you can modify objects after enveloping.
When you add a point to an enveloped object, normally it becomes automatically weighted based on the surrounding points. It is better to add new points before removing old ones — this means that there is more weight information for the new points.
In some circumstances, the new points aren't automatically weighted. In these cases, you can assign deformers manually as described in Changing Deformer Assignments Manually.
Deforming and Moving Points on Envelopes
If you want to apply a deformation or move points on an enveloped object, make sure to first set the construction mode based on what you want to accomplish. For example:
If you want to modify the base shape of the envelope, set the construction mode to Modeling.
If you want to author shape keys on top of the envelope, for example, to create muscle bulges, set the construction mode to Secondary Shape Modeling.
For more information about construction modes in general, see Construction Modes and Regions [Modeling and Deformation Basics].
For more information about construction modes and shapes, see Creating Shapes in the Shape Modeling Construction Modes [Shape Animation].
If you move a point above an Envelope operator and do not bake the resulting MoveComponent operator into a shape, the result may not be correct if you later move the deformers. This is because the MovePoint operator stores data as an offset from the object center, and does not take into account the envelope deformation.
To solve this problem, make sure that Modify 
Component Relative Mode is on before moving points on an envelope. For more information, see Relative Mode [Modeling and Deformation Basics].
Envelopes behave differently from other objects when you transform them:
If you try to scale, rotate, or translate an envelope, it appears to be unaffected. This is because all of its points are assigned to the stationary deformers.
If Centers are visible in a 3D view, you can see that the envelope is "really" being transformed.
If you transform an envelope in Center mode, it appears to react in the opposite way. This is because transforming a center actually applies a compensation to the object's points.
In general, it is not recommended to transform an envelope because you may get strange results if you later reset the reference pose. Instead, you can put the envelope and deformers in the same hierarchy or model and then transform the whole tree.
