One of the main differences (and the most significant) with CECL is the Life of Loan concept, where you must estimate both probable and projected portfolio losses over the portfolio life. The complexities of Life of Loan arise when you have to anticipate repayment and the rate of prepayment.
PCBB's white paper, Loan Maturity and CECL: The Balance Between Rollover Risk and Reserves, examines loan maturity and the balance between short and long-term loans to help you prepare for CECL.