Ok, I have booking from WNZ (MU) to HKG (CX) and HKG (CX) to JFK (CX) on the same date. I decided rather than rush home to USA I will stay overnight in HKG to enjoy the city for one night. So I called MPC to ask for the change. The lady helping me put me on hold for literally 10 minutes (no doubt she's a rookie and asking her more experienced collegue for help on this simple request), and then she returned and says: "we can change it for you but you need to pay $150 USD 'transit fee'..." I was like WHAT? I've never heard of this "transit fee" before, so I asked her to explain that one to me.

She says a transit fee is when you delay in your transit with CX airlines. The ticket is MU --> CX on the same date, and because I wanted to delay a day that CX will charge me $150 USD. I was like er.... NO.

Called my travel agent back in the States instead, get him to change my date, done. Call back MPC to make sure it's done, and yes it's done (with the agent obviously annoyed that I would call just to confirm a confirmed ticket)

Anyway, what the hell is 'transit fee'?

This is something new to me. It does not seem to have anything to do with X or O on the ticket.

I suggest you do OLCI. Even though the booking was changed, it does not necessarily mean that there is no cost associated with the change. If you can successfully do OLCI, then you are likely all set.

Could this be because the ticket is issued on MU stock and this is the fee that CX charges for "revalidating " when making changes to tickets on other airlines' stock?

But that handling fee costs approx 40 USD.
HKG airport tax is charged for non-same day transit within 24-hours.
But, these charges can not explain 150 USD.

One possibility is the original fare permits only same day transit.
Some fare only permits same day transit. For that type of fare, if you want to make non-same day transit, a different fare will be applied.

Not sure, but it would seen strange that my agent would by the ticket on MU stock when the bulk of my travel is on CX (JFK-HKG-TPE-HKG-WNZ-HKG-JFK, only to and from WNZ is MU)

The sum of USD $150 rings a bell.

This is the same amount as a change fee on Internet fares (for example changing a return leg of a flight when on a business class "I" fare)

If I am right, your bulk ticket only allow one or two stop-over(s) in Hong Kong. On your last leg, you properly used all teh allowance already.If you wish to stayover in Hong Kong again, as your fare penalty, USD$150 will be collected as reissue. It is all to do with fare restrictions. Most of the return fares allow one stop over in Hong Kong.

At this moment your fare is WNZ--X HKG--JFK. If you change it to WNZ -- HKG --JFK, you have to pay the penalty.

The problem is your fare is X US fare. The Hong kong agent needs to refer the X US fare bucket to find the fare resitriction. It is common for staff to read the fare through before they can speak to you. There is nothing to do with new staff nor old staff. Even experienced staff, facing so many promotional fares in different seasons, it is nature for them to have a double check before they speak unless iyou are booked in fully fexible fare e.g. FCY... You always check before you make the change. It is unnecessary to make a fuss about staff. They are just doing their job in the correct way.

Good post, but it's wrong to my situation. I spent no stop over in HKG prior, it's a straight go to TPE from JFK via HKG. So the staff, as careful as you have given her all the doubt in the world, is wrong as well.

Perhaps your fare does not allow a stopover at all ?

Nope, like I said a quick phone call back to my TA solved it just like that, and I get to spend an enjoyable night in Hong Kong.