How to calculate the different of upgrading air ticket to higher-class for return portion
For example
A passenger holding an economy 2-months ticket for sector
PAR SV JED SV PAR
Passenger traveled sector PAR IY SAH on 25Jul10
On 20Aug passenger reported to airline office asking
To upgrade his return sector JED SV PAR to C/class.
Question How to calculate the different???
Is it must to go back to PAR fare on date of ticket issuance or
I can apply JED SV PAR current fare and add it to the balance of ticket amount, pls advise the rule and reference number if possible with thanks