How to change default fee token
You can check default fee token order on-chain, go to Polkadot Webapp - Select the chain (Acala or Karura)
On Acala, the order might be ACA > aUSD > LCDOT > DOT.
If a user has no ACA balance, then aUSD will automatically be used as fee token.
Users can set their next default fee token to other tokens by executing the following transaction: