There's only one thing that will do it - the Dalai Lama says he was wrong and recants every negative thing he's said about Dorje Shugden.
I can't see it happening any other way, otherwise, even if people are 'allowed' to practise again, Dorje Shugden will always have the stigma of being a worldly spirit and those who practise will be seen as non-Buddhists.
I have no doubts whatsoever that the ban will go down eventually when:
(a) The Dalai Lama goes into clear light;
(b) The Dalai Lama reverses the ban, either admitting that he was wrong or providing a sound enough reason why he imposed the ban which he is then reversing;
(c) the CTA ignores the ban and publicly declares that persecution based on religion is unlawful and passes a strong resolution to prosecute those found guilty (seeing that they cannot pass a law to remove the ban since the ban was never passed as a legal act of parliament);
(d) China and the CTA comes to a compromise and Tibet is "returned" to the government of the CTA as an autonomous region within China. (when this happens China is unlikely to allow to the Dalai Lama's dictate to apply to Chinese citizens practicing Dorje Shugden). Whichever occurs first.
What would cause the ban to be removed faster would be faithful Dorje Shugden practitioners around the world pushing hard to create awareness of the unjust ban and bringing about situations where high profile people like powerful political figures, well respected scholars, media personalities with large followings, famous actors, human rights activist, and such like question the ban and call for the CTA to answer. Unfortunately, at the moment the ban has not caught sufficient attention on a global scale and is seen as an issue within the family of Tibetan Buddhist practitioners. Spreading the practice explosively is a precursor to getting the attention and support of influential people around the world.
When the ban does come down, I don't think Dorje Shugden will have a stigma at all. Not when the high lamas and well respected people within the tibetan Buddhist community come out openly to declare that they too are practitioners of Dorje Shugden. Moreover, we only need to look at the Yamantaka practice now, remembering that Yamantaka too was taught of as a demon when Ra Lotsawa first introduced it.