Very interesting - my gut tells me you lucked out. Given that the reservation certainly doesn't have a "not valid after" date, and the ticket is the only place that has the "not valid after" date (column with a date for each segment), if your ticket has a new April date instead of the original Dec date then there is nothing to say you need to use your ticket by Dec 05. I may be wrong, but if not we should all find out who re-issued your ticket