The last day to redeem for Star Alliance awards using BMI Diamond Club miles is May 31st, 2012 that is this coming Thursday. You cannot voluntarily amend any bookings after this date. Most likely they will have some sort of facility in place for involuntarily changes when an airline drops the route or changes the flight times drastically.
I would encourage trying to get the bookings in as early in the week or this weekend as possible. Back in April before the Lufthansa group of airlines became unavailable for award bookings, it was borderline impossible to get through in the last two days. This will probably happen again this time around as well.
I will probably spend this weekend trying to get my travel plans together for the rest of the year. There will be some trips where using BMI Diamond Club miles would be the best choice.