Hi all

Long time lurker, first time poster! Wasn’t able to find a thread covering this via the search, but apologies if I have missed the answer elsewhere in the forum.

My current situation:

• I have the return leg of an LHR-HKG-LHR trip currently open and booked for December in WT+ (fare bucket T). ​​​​​​
• My plans have changed and I need to move the date of the flight - either to later in December or to September.
• I also want to use Avios to upgrade to CW, but there is no availability on my current travel date.
• There is no change fee associated with the booking as the Covid rules apply to it.
• ​​​​​​​To change to booking to my new preferred date in December would cost £540 in additional fare, and to change it to my preferred date in September would cost £430 in additional fare. There is award space in CW on both dates, so I would be able to upgrade.
• ​​​​​​​To change the booking to a random date in October (which also has reward availability in CW) would cost £209 in additional fare.

​​​​​
My question:

• Given that changing my booking would not incur a change fee, could I change the date of the booking to the October, paying the £209 in additional fare, upgrade to CW using Avios and then change the date of the flight again to one of my preferred dates in September or December without incurring further cost?
  Or when changing the booking would I be required to pay the difference the original underlying T fare to the new underlying T fare?

thanks in advance for your advice!