The introduction of business rules allows both new business rules calculations and existing custom calculations to co-exist and be used in Cost Estimate (CE) and Compensation Calculation (CC) templates and expense/invoice auditing rules. If you have existing custom calculations that are fully operational, it is recommended that these are left unchanged; consider using the new business rules for new or existing but complex / troublesome cases only.
There is no simple way to convert existing custom calculations to using the business rules because they are very different. To start creating business rule, you can check how to add a new business rule calculation or how can I start creating Business Rules.