Reschedule fees charged double the price of new booking (Not informed by customer officer on the better option)

Dear Sir/Mam,
Last Sunday i called customer care for rescheduling our tickets (In view of Covid-19). 
Before the call to customer care, I did not check the fees for totally new booking (not reschedule). During the processing of re-booking, the customer care officer just provided me the change booking fees (which was in total was ~436 SGD as additional charge) and i allow them to proceed without any further discussion/check on what will be the fees for totally new booking (assuming the officer would have checked and offered me the best option for rescheduling). Same time i did reschedule for another booking for same flight (Booking ID# CLTWTN) with new additional charge of ~141 SGD. 
After the settlements of payment, couple of days later, i realized the new booking for the same flight & date is at half the price of change fees i paid (i.e. 436 SGD vs ~260SGD for new booking). Upon discussion with the customer care officer on very same day of realization, I was given feedback that i should have checked the prices for new booking before i allowed customer care officer to proceed..With this type of discussion, I felt like this is not really professional from airlines. 
Also other thing i realized is that the delta paid for booking of 2 person (under booking ID# CLTWTN, additional amount paid ~141 SGD) and 4 person (under booking ID# MFNS9C, additional amount paid ~436 SGD) which i cant understand.
On separate note, I received one email for free date change voucher from Jet star after all this. So I would like to know how this will help me as well.

Indonesia-Denpasar (Bali)