This may be common knowledge or it may be a one off thing but I had Angemon then I lost a fight (piximon I think) and he turned into devimon (this is kind of weird as they're both champion level). Devimon was way more powerful than angemon but unfortunately this did mean losing a heart so he died at only 14 days. If this doesn't work for you please accept my apologies, it may have been something else that triggered it e.g. Dissatisfaction.
Hope this helps!
Added by: Joe the Lion Jul 18th 2008, ID#15341