[More Cowbell]

From what I understand, if the coach that signs them leaves, they can withdraw their LOI.

No, it's not a given that a recruit can withdraw his LOI. The school reserves the right to release the player or not, although they almost always do.

The whole LOI system is a disadvantage to the recruits, which is why this issue was mentioned in regards to Calipari's Memphis recruits, specifically Xavier Henry. As has been widely reported, Henry's LOI contained a special clause that allowed him to withdraw the letter if Calipari left. Without this clause present in a LOI, the university holds all the cards.