Just to add to what Phil said, although you can't make the change online, you can check for availability and be armed with the dates when you call up to change. Just start the process of a one way CPT-LHR Club booking with miles for 2 people via the Exec Club pages and you'll see a calendar of all (or none!) of the dates available with 2 J award seats that month.

Also, they will always let you go 6 months beyond your origonal expiry date so I wouldn't worry too much about that.