The
T&C do not seem that vague to me:
− Rebooking is, provided the points are still valid, allowed up to one (1) working day before the offer in question must be used (expired points cannot be reused). A service charge is incurred by rebooking.
– Points cannot be transferred back after a bonus trip has commenced, and the return trip cannot be rebooked or refunded.
− In exchange for a service charge, offers can be cancelled up to one (1) working day before they should have been used. Only valid points are transferred back to the member’s account (expired points cannot be reused).
You will need to call anyway to book on LH/OS. Why not ask them about the rules, to be sure?