The rule behind the rule
Your elections are locked for the plan year. That is not your employer being difficult — it is a condition of paying premiums with pre-tax dollars under a Section 125 plan. The exceptions are set by regulation, and a change has to be both a listed event and consistent with it. A birth lets you add the baby; it does not let you switch from the HMO to the PPO because you have decided you prefer it.
You have thirty days from the date of the event. Some events allow sixty. Report it the week it happens and the question never arises.

