If an existing 457(f) plan has a rabbi trust and thus the substantial risk of forfeiture was being subject to creditors, how should it be amended to comply with 409A?

You have got to be kidding if you are suggesting that the credit risk was a substantial risk of forfeiture for section 457(f) purposes. If you are not kidding, you have nothing to worry about for section 409A compliance because all of the accrued amounts are already taxable.