can region be made static after applying wiggle command

Hi Christoph!
I try to make my question clear.
In the group command for region style ,it is written that
"The region style puts all the atoms in the region volume into the group.Note that this is a static one-time assignment.The atoms remain assigned(or not assigned) to the group even in they later move out of the region volume."
In my simulaton i poured particles into whole region of a box,but i want to know different features (vel,position etc) of particles present only in the centeral region of this box at different time steps .Hence i grouped the central region of box as a grp2.The atoms initially present in this region are assigned to this region(grp2), let they are 200.When i apply the wiggle command,the atoms leave this region (means the number of atoms changes in this region).But when i dump the centeral region(grp2),in the dump file the number of atoms for the centeral region remains fixed (remains 200) as initially assigned to this region and do not chnge with time.If i want to delete this region after dumping ,this is again not allowed.
i hope you will help me.Thanks
sajjad