Yes, changes before departure will result in the whole ticket being repriced at today's prices. It "should" be cheaper to amend the return after flying the outbound depending on which date's fare basis they use to reprice the return - it should be based on the date of purchase, see:

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

But also depending on the fare rules applicable the penalties may be still be high.

In my recent experience (September) you may have to call in to do this on an open jaw ticket after departure rather than being able to change it online.