After you have applied an envelope, you can add and remove deformers.
When you add deformers to an envelope, you have the option of recalculating the original initial weights. If the weights are recalculated, any modifications you made to the weights and deformer assignments are preserved.
Choose Deform 
Envelope Set Envelope from the Animate toolbar. If there is already an envelope deformation on the object, the Envelope dialog box opens.
If you do not want to recalculate the initial weight assignments, leave the Automatically Reassign Envelope When Adding Deformers option off. The new deformers are added to the list of deformers, but no points are assigned to them automatically. You can assign points to deformers manually as described in Reassigning Points to Specific Deformers.
If you do want to recalculate weights, turn this option on. The initial weight assignments of all the points on the envelope is recalculated and any modifications you have made are reapplied on top of the new initial weights.
Note that this option has no effect on envelopes that have been frozen. Envelope weights cannot be automatically recalculated in this case.
If you have modified weights manually, you should leave this option off. Otherwise, the result of the new initial weighting combined with the manual modifications will probably not give good results.
If the envelope weights have been frozen, this option makes no difference. Envelope weights cannot be automatically reassigned if the Automatic Envelope Assignment operator has been removed.
Pick the objects that will act as new deformers. Left-click to pick individual objects, and middle-click to pick branches.
When you have finished picking deformers, right-click to terminate the picking session.
After you remove a deformer, the envelope is no longer affected when the deformer is moved. The weights of points that were assigned to the removed deformer are normalized among the remaining deformers so that the total weights remain 100.
