It is possible if the price of the flight is exactly same, you have to change the go and return flight, deselect luggage for the going flight, select luggage for the return flight.

For my case, Penang -> Langkawi, Langkawi -> Penang
2 persons

Initially......
Penang -> Langkawi [Going]
Flight @ price A, with seat selection +RM5x2, without check-in baggage.

Langkawi -> Penang [Return]
Flight @ price A, with seat selection +RM5x2, without check-in baggage.

But then......
Penang -> Langkawi [Going]
Want to change the flight option but the price had increased from A to B, so not worth it la...
So, remains unchanged.

Langkawi -> Penang [Return]
Flight still @ price A, without seat selection -RM5x2, check-in baggage +RM20

Total amount due : RM10.00

I hope you understand what I am saying, I not so sure if I explain it well enough for you guys to understand, kinda blur myself too