If I have set credit type as One-time credit and allocated credits to Subaccount, can I add/subtract credits later on?

Yes, Master account can revise the credit balance of Subaccount at any point by following below steps:

• Click on ‘Add/subtract Credits’ to increase or decrease credit balance of Subaccount.
  • Select ‘Add’ if you want to increase credit balance,
  • Select ‘Subtract’ if you want to decrease credit balance.
  • Enter amount of credit you want to increase/decrease from the available credit balance.
  • Click on ‘Save’