I would use this UIViewController's method:
- (void)willAnimateRotationToInterfaceOrientation:(UIInterfaceOrientation)toInterfaceOrientation duration:(NSTimeInterval)duration;
that is called upon rotation, and call -setNeedsUpdateConstraints
from there.
If you need to do additional calculations or manage the contrainsts add
- (void)updateViewConstraints
{
[super updateViewConstraints];
// do calculations if needed, like set constants
}