A lock core assembly. The assembly includes an external cylinder, an inner cylinder rotatably fitted in the external cylinder, alignment elements provided in the inner cylinder, at least a limiting element provided in the external cylinder able to engage with the alignment elements, and a key to be inserted into the inner cylinder to move the alignment elements to an unlocking state with the limiting element. When the engaging member is detached, the limiting element is separated from the external cylinder, and then the original coupling of the key and the limiting element are substituted by another coupling of a key and a limiting element, such that, when a user loses his key, he can have the lock core assembly repaired without changing the entire lock set.