Another plan has some extra services that I can't get in my own plan. When is the next time I can change plans?
You can change plans during the annual FEHB Open Season and whenever you have a qualifying life events (QLE) -- such as marriage. Becoming aware of another plan that has better benefits, even if you didn't expect to want the extra benefits when you had a chance to change plans before, does not qualify as a "QLE" that allows you to change plans.