To stop wearing hijab is haram. Hijab is obligatory. Moreover, one has to cover her hair and the entire body except the face and hands upto the wrists (wrists to be covered). The clothes should be in such a way that the body parts (contours) shouldn't appear and (the clothes) shouldn't be considered as an adornment.
Thus, if the clothes which are worn instead of veil cover the body in the above mentioned condition, there is no problem in it. However, it is better for a Muslim lady to wear
veil (chaadar) as her hijab.
Ayat...Sistani's Office, Qum