Why can’t I change my Player Name and Guild Name in the Grand Battle?

The guilds and guild members who can participate in the Grand Battle are decided at the time rankings are calculated.
Once rankings have been calculated, Player Names and Guild Names will be set, and any name changes made afterwards will not be applied to the Grand Battle for the duration of that season.

While name changes will not be applied to the Grand Battle, they will be applied normally to other in-game content.
However, this may cause your Player Name or Guild Name to appear differently in and out of the Grand Battle.