Regarding the situation where the use and enjoyment of a joint property is granted by the court to one of the spouses who later shares the property with a new partner, the question of whether the circumstances require a change in the use and enjoyment of the property would seem to depend on whether or not there are children involved.
Where there are no children, the use and enjoyment of the property will have been conceded to the particular spouse because the court will have evaluated the situation and decided that this spouse was more deserving of protection than the other.
Simply put, once that spouse is living with a new partner, the new circumstances may well be sufficient for the judge to consider, upon a request made by the former spouse and (at least part) owner of the property, that the situation that gave rise to the need for greater protection no longer exists and that the property should be returned to the former spouse or sold.
If, on the other hand there are children, the use and enjoyment is in reality indirect, as the use is really in favour of the children. Accordingly, should the spouse granted use and enjoyment of the property decide to share it with a new partner, this would not extinguish the rights enjoyed by the children.