You mean as far as would they lose their knowledge? The answer would be no, and there would be an expectation that they would more quickly pick up Magisteria's tech or magic. Eventually, their canon powers and the game-given powers would be brought into alignment.
For example, a fire bender would likely quickly pick up the game's fire magic, and then when and as they get access to canon powers, they would be an especially OP fire mage, as they would be basically double-dipping. But of course, precisely because they have that covered, a player might want to have them also pick up other forms of magic, or focus on doing something different and non-conventional with the abilities. And of course they could also pursue the tech path and go down a different path entirely, perhaps choosing to make mecha suits which they wear while bending. This would not be considered playing both sides, but rather picking up a new discipline. However, they would not be able to pick up new magic in game due to the attitudes of those who would train them.
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For example, a fire bender would likely quickly pick up the game's fire magic, and then when and as they get access to canon powers, they would be an especially OP fire mage, as they would be basically double-dipping. But of course, precisely because they have that covered, a player might want to have them also pick up other forms of magic, or focus on doing something different and non-conventional with the abilities. And of course they could also pursue the tech path and go down a different path entirely, perhaps choosing to make mecha suits which they wear while bending. This would not be considered playing both sides, but rather picking up a new discipline. However, they would not be able to pick up new magic in game due to the attitudes of those who would train them.