Has anyone experienced a problem with waitlisting for "married segments"?

I have purchased a ticket for TPE / HKG / LHR / HKG / TPE in D Class. The ticket can be rebooked without any penalty. The outbound flights are CX565 on Day 0 and then CX253 on Day 1.

Now I want to change the outbound flights to CX565 on Day -5 and then CX253 on Day -4. The agent at first told me that D Class was available for both flights so no problem, but he later said he could not change the flights which I guessed (and he confirmed) was because of "married segments" issues. (Expert Flyer was showing high availability in D Class for CX565 on Day -5 but low availability in D Class for CX253 on Day -4.)

So I asked to be put on the waitlist for CX565 on Day -5 and then CX253 on Day -4. After being put on hold for more than 20 minutes, the agent said he could not do it and neither could his colleagues. He said that, in order to waitlist, I would have to cancel the flights for the old dates first or else change the flights only once the new dates were available. In other words I could not waitlist in the normal sense.

What I would have to do, the agent explained, was to call back regularly to check availability, which would be a pain to say the least.

Please can anyone shed some light on this problem?