Hollows are a staple of changeling life. They provide a safe haven in the ever-shifting maze of the hedge, free of the beasts and monsters that stalk its paths. In time, though, every hollow loses its usefulness. The changelings who made it die or move away, they lose access to its doors, etc. Since creating and maintaining a hollow requires XP to purchase the Hollow merit, what happens to that XP?
When a hollow is abandoned, as is often the case, the XP invested in its features is lost. The hollow will take some time to decay based on the number of merit dots that were used on it.
If a group of changelings wishes to get back their investment, they can spend the time to break down their old hollow themselves. This takes an hour of work for each dot of Hollow merits related to the space. Everyone who helps break down the hollow splits those hours. To reclaim their spent XP, a changeling must meaningfully help deconstruct the hollow. That means assisting for at least one hour, probably more.
Once the hollow is fully destroyed, each character removes the related Hollow merit from their sheet. The XP each had spent on that merit will come back within the next session or two.
For example, a motley of four changelings wants to deconstruct their old hollow. The hollow has Size 3, Wards 4, Amenities 2, and Doors 3, giving it 12 total dots. With all four of them helping, it will take 3 hours of work to take down the hollow. After those three hours, all four changelings remove the old hollow merit from their sheet and regain the XP that they personally spent on those dots.
Each character can only regain the XP that they personally spent on the Hollow. Other changelings without a stake in the hollow can help destroy it to speed up the process, but will not receive any XP for doing so.
Reclaiming a partially decayed hollow works in a similar way. The GM decides which dots have been lost and those are removed from the character sheets. Any remaining dots can be reclaimed as normal or used as part of a new hollow.
If one or more characters cannot take part in the reclamation, the changelings present can choose to stop before the hollow is completely gone. The missing characters can then return at a later time and finish the job.