In my bathroom, there is an extractor fan mounted in the plasterboard/drywall ceiling. It's there to remove moisture and stale air. Naturally, there is a hole through which the fan fits, and the fan is held in place by four clips that radiate outwards, over the edge of the hole, from the internal (roof-space) edge of the fan.
The edges of the hole are now crumbling and the fan is in imminent danger of falling through the hole. The crumbling extends about 10 mm outwards from the edge of the hole. An additional problem is that the fan needs to be replaced and it isn't always possible to get the exact size for which the hole was cut, although it is probably possible to get something within 10 mm diameter of the existing fan.
How can I repair and strengthen the inner crumbling edge of the hole without replacing vast amounts of the ceiling plaster. As the end of the task, I want a hole into which I can mount a new extractor fan.