Les Mills has a HUGE way to re-launch (w/ online support, a big old re-launch guide and support) - and your GFM should be in contact w/ your local club coach for great ideas.
entire point of re-launch event is to get people excited for the newest release, to draw additional member & guest traffic into the club (i.e. SALES!) - and to breathe new life into the program for another quarter. Unless you have a very specifically planned "sneak peak" or VIP class where you'll show a bit of the new release to a select # of VIP members/guest/media etc prior to the event (goal being to raise more excitement, raise event awareness, get press coverage etc) - then doing it before does spoil the point of re-launch.
Finally - your team should be practicing outside of a regular class setting. Guinea pigs are your other instructors and staff members - NOT YOUR CLUB MEMBERS. Their first view and all views of the newest release should be World-Class amazing, not a "practice" - needs to be 100% sorted b4 members ever see it. W/ BS #77 - more chorey to work on vs. some of recent releases, so they'll need to be sure eve more than normal, making sure they've had adequate practice/prep time to be spot -on.
Do a dress rehearsal w/ your staff to show them the new release, get feedback on your performance, and to get a good trial of the stuff - gets them excited about it and gives you practice - double benefits.
DEFINITELY get in contact w/ your local club coach for more info :)
you can also ask in the GFM forum for more ideas/thoughts on this (even though you're talking BS specific)
good luck!
B