Once you connect a custodial account—for example, an Uphold, Gemini, or bitFlyer account—to your Brave Rewards profile, you won't be able to switch to another provider on that profile. However, you can always start a new Brave Rewards profile, then connect your new Brave Rewards profile to an account from a different custodial account provider.
You can create a new Brave Rewards profile by resetting Brave Rewards. You can reset Brave Rewards by going to brave://rewards, then pressing "Reset". Learn more about resetting Rewards, here.