In order to change your Primary Shift Account, head into “Settings”, “General” scroll to the bottom and select “Reset App Data”. Click “Agree” and your Shift account will be reset.
You will need to then re-add all of the accounts you would like to use with Shift. Follow the same steps you did when first creating a Shift profile. At the end, you will be given the chance to choose the Primary Shift Account you would like - this is where you will switch it to the account you prefer.