It probably depends on the scale. If you try to form a new in-group identity over a short time in a smallish group against a background that reinforces the existing categories, then I can believe the effects will be temporary. However, if the group is larger, the background identities weakened and de-emphasized, and enough time given, I think it'd work.

My impression is that current "anti-discrimination" attitudes actually reinforce the existing categories, even more strongly and explicitly than had been done before. There's a lot more labeling and "reasoning" through labels going on in places where it seems very counterproductive.