No you can't. Came as a rude shock to me as I was working on a redemption on 31 March, trying to avoid April YQ rises. Must be done over the phone, and won't take effect until 24 hrs:
https://www.cathaypacific.com/cx/en_...e-service.html
Suspension of online service
We are currently upgrading our system, and the online service for changing redemption group members is temporarily suspended during this transition. If you need further assistance, please contact the worldwide toll-free Marco Polo Club Service Line on +800 2747 5500 or email us using your registered email with The Marco Polo Club.
During this period, a service fee at USD90 will be charged for every change or deletion of existing redemption group members, regardless of how many changes take place.
We apologise for any inconvenience this may have caused. Thank you for your patience and understanding.