The free changes part only applies to whether the change fee shown in the fare rules will be charged or not. You will still need to pay any fare difference.

Changes made after flying the outbound should be priced at the fares applicable on the day the ticket was originally booked. But that does not mean that you will not pay more as the fare bucket you originally booked into may no longer be available when you make the change.

Have a look at the "Changes (Reissuance)" section here, it details all the technical bits and gives a few examples:

https://www.qatarairways.com/tradepo...d-Charges.html