score:3
Yes and no.
You can (almost) always change your ticket, however in many cases this will be prohibitively expensive. The exact details depend on the terms & conditions of the specific ticket that you have already booked.
For most ticket types a change will cost a change fee plus or minus the difference in the ticket prices. Before you call, you should
Unfortunately it's not unusual for phone agents to give information that's sub-optimal (for you) or outright wrong. Preparing can be helpful and sometimes you can get a different answer from a different agent.
Case in point: I called twice just last week about the same ticket change and the two answers differed by over $350.