I have also called to "Modify" my reservation by changing room numbers. Yes, some agents try to reprice the reservation, but I ask for a Supervisor or an experienced agent who never have a problem completing the modification and not charging me. The only change I found is that the mobile APP only displays the reservation, any changes or cancellations have to be done by an agent.
Having changed several rooms of my own, and having read hundreds of change threads here on the forum, I don't think the agent is going out of their way to
cause a reprice. I think the reservations system is simply repricing the ticket by default anytime a room is changed. At which point the agent is naturally reluctant to modify the price the computer says is correct. Can you really blame them? I'd be reluctant as well. The only problem I have with it is that they are sometimes extremely resistant to involve a manager, even when asked directly by the passenger, and when that happens I can no longer defend their actions (or lack thereof).