I think it’s a fairly common problem. Whenever I try to change a return leg back to TPE online, instead of flight dates loading, I get a “We are experiencing technical difficulties” message. So the only option I have is to call MPC. They will change the booking manually and won’t be able to provide web support for what appears to be a technical glitch (at least in my case).

When working correctly, alternative dates and times of flights should load and if money needs to be paid (I.e. if the ticket requires a flight change fee or if surcharges have increased), the payment option is given.

It should be easy but it’s frustrating when it doesn’t work.