A set is exposed the moment it's completed by calling a discard, or when a player chooses to reveal it.
Whether by call or by choice, once a set is on the table it's final — it can't be broken apart, hidden, or swapped for a different combination later in the hand.
Example
If you call a discard to complete a Kong of 2 Bams, that Kong stays exposed even if you later draw a tile that would have let you use those tiles differently.
