You have a wealth of knowledge in your mind that remains buried most of the time. You could be an amnesiac, haunted by ghosts, or anything else that might cause you to have memories come and go.

During a Short Rest - Self

At the end of a short or long rest, you can swap one of your skill proficiencies for a different one. If you had expertise with the old skill for any reason, you have expertise with the new chosen skill instead.