It is possible that the cheaper fares have sold out since you booked.

Also be aware that with many airlines, if you amend either direction of a return ticket before flying the outbound, the whole journey outbound and return is repriced at the fares available on the day you make the change. So it is likely your whole ticket is being repriced in both directions.

It is usually cheaper to change the return after flying the outbound, when the amended return journey will be priced at the fare available on the day you made the booking (subject to fare bucket availability, plus any change fee). This will be stated in the detailed fare rules

And as per the other thread(s), if a passenger chooses to book a cheaper ticket with a long connection when there is a shorter but more expensive connection available, the subsistence costs of that chosen longer connection are on the passenger. The airline didn't force anyone to make that choice.