Improvement for the Christmas Market has the cost of this building upgrade is ...
It should follow the same rule has the Epic yard buildings when destroyed it will keep the upgrades.
Since Present cannot be kept for next christmas it make this upgrade extremely costly and having a time constraint to actually find the right and appropriate location.
I do feel like the upgrade for this building should be kept after you destroy it (Move it to star) like other really expensive upgraded building (Epic Yard) I am not counting pop token has those are costly and you lose the upgrade it should keep them and only lose the upgrade if you actually trade the building.
Building like Christmas Market should keep the upgraded functionality after it has been moved to star.