You have the choice to change your team before any battle starts in Pokemon Masters. You are allowed to choose from three Sync Pairs to fight your battle with, so in order to change your Pokemon in the game, you will have to swap the Sync Pairs as you are not able to change your Pokemon beyond your chosen Sync Pairs.
Change Pokemon in Pokemon Masters
You will not be able to find any individual Pokemon out there in the wild as the game defines Sync Pairs for Pokemon. The only changes you are allowed to make to these Sync Pairs is by evolving the Pokemon in them, given that they have this option.
The player can view this option by checking the Dex section in their Polyphone by selecting any one of the pairs that are shown here which include the Pokemon and their Trainer.
If you look below the listed Pokemon, you will be able to see how many times you are allowed to evolve these Pokemon.
For now, there are sixty-five Sync Pairs listed in the game, although more are going to come out soon. All new players have Pikachu as their starter Pokemon, and the game does not allow you to change your starter Pokemon as of yet.
Conclusively, you cannot change the Pokemon in Sync Pairs either because the Trainers and Pokemon in these have synced attacks.
But you can still look for Sygna Suit versions for particular Sync Pairs which will help you change the Pokemon. For instance, the Sygna Suit Brock gets you Tyranitar instead of Onyx.