Guild Wars 2

The mordrem fight shown in the trailer looked amazing, but is this only for the first time a guild is able to beat them?
I don’t know about most player but the guild I’m in is a multi-timezone guild. It’s more than likely that it would be run in an inaccessible time for myself(as well as others) or as an unscheduled whim of a leader.
Will there be an option to rerun the assault(without flattening the progress of the guild’s hall) like a personal story instance?

It’d be interesting if, like the Silverwastes, it was able to be overrun due to guild inactivity thus forcing the guild to reclaim it.