If you book a special-offer such as IGT, OGT or VGT -- no changes. You can upgrade at port pending availability, but I do not believe you can upgrade prior.
If you book a "regular" GTY -- such as 8A GTY or 5B GTY -- those can be changed without penalty at any point prior to PIF. You can upgrade to a higher category, downgrade to a lower category, remove guests or add guests (based on availability). Anything after the PIF date will incur penalty based on how close to the sail date. You can upgrade at port.
Enjoy your cruise!