You cannot change the account code. If the account is linked to a department, and there are transactions for it, you cannot change the department field. Everything else can be changed.
If the G.S.T. status of the account is changed transactions entered after the change have G.S.T. calculated based on that change. To change existing transactions to reflect the new G.S.T. situation edit them.
If an account is made inactive no further transactions can be entered for that account.