Some help & advice needed:

I booked two return premium economy redemption tickets for travel from HKG to MLE for travel 24 May / 31 May, and now wish to upgrade to business with points. I called MPC and was told that it's possible for USD100 per ticket to rebook + 26k miles. However currently there are no redemption seats available for the return leg. I also saw on their website that waitlist are available for the return date. Is it worth while being placed on waitlist? The call centre also mentioned that I will be charged rebook fee twice, first the outbound leg change to business and a second time if I get the return leg upgrade. Is this right?

Seems an it odd that Cathay is not willing to take my money and points...