Passage: Final Fantasy X introduces the Conditional Turn-Based Battle system in place of the series' traditional Active Time Battle system first used in Final Fantasy IV. Whereas the ATB concept features real-time elements, the CTB system is a turn-based format that pauses the battle during each of the player's turns. Thus, the CTB design allows the player to select an action without time pressure. A graphical timeline along the upper-right side of the screen details who will be receiving turns next, and how various actions taken will affect the subsequent order of turns. The ordering of turns can be affected by a number of spells, items, and abilities that inflict status effects upon the controlled characters or the enemies. The player can control up to three characters in battle, though a swapping system allows the player to replace them with a character outside the active party at any time. "Limit Breaks", highly damaging special attacks, reappear in Final Fantasy X as "Overdrives". In this new incarnation of the feature, most of the techniques are interactive, requiring button inputs to increase their effectiveness. While initially the Overdrives can be used when the character receives a significant amount of damage, the player is able to modify the requirements to unlock them.Final Fantasy X introduces an overhaul of the summoning system employed in previous games of the series. Whereas in previous titles a summoned creature would arrive, perform one action, and then depart, the "Aeons" of Final Fantasy X arrive and entirely replace the battle party, fighting in their place until either the aeon wins the battle, is defeated itself, or is dismissed by the player. Aeons have their own statistics, commands, special attacks, spells, and Overdrives. The player acquires five aeons over the course of the game through the completion of Cloister of Trials puzzles, but three additional aeons can be obtained by completing various side-quests.

Question: what time can a player be replaced?
any time.