Changing sides refers to switching one's stance or allegiance from one side to another. The antonyms or opposite words for 'changing sides' would depend on the context in which the phrase is used.
If the phrase is used in the context of sports or games, the antonyms could include words like 'staying loyal', 'remaining steadfast', or 'keeping consistent'. In the context of politics or relationships, antonyms could include 'staying true to one's beliefs', 'maintaining loyalty', or 'upholding values'.
In general, the opposite of changing sides would involve holding one's ground, sticking to one's principles, and maintaining consistency in one's actions and beliefs.