Has anyone who has gone through the Vancouver consulate needed to push back the date of their interview past the four month NOA2 window?

Our NOA2 is good until 2/12/07, and while we'll have the paperwork in by then, we may need to be more flexible with the interview date. My fiancé's job keeps him away from his home for weeks on end, so requesting time off well in advance to schedule both the interview and the medical would be necessary. Vancouver seems to schedule interviews about four weeks after you send the checklist, and if we get in the paperwork by December or January, we'd be cutting it close.

Anyone have experience with something like this? Did they automatically extend the petition validity? Did you need to request it in writing?