The interview letter should have a section enabling the applicant to request postponement . Didn’t do it myself but I vaguely remember the section . Preplanned travel is a reasonable basis . However, there’s no choice or expectation as to when the rescheduled interview would be . There’s a procedural formality enabling the applicant to make an infopass appointment if they don’t get a rescheduled date within a given timeframe (3 months from original date ?), but that’s it .