There is no requirement to transition from previously configured custom calculations to the new business rule calculations.
All legacy custom calculations remain unaffected with the introduction of Business Rules which means that it is possible and perfectly fine to use the new method to calculate one or two line items on a Cost Estimate template and have the other line items using the legacy custom calculations. These calculations are found on the Company Calculations screen at the Company level.
Business Rules are found on the Business Rules screen at both the Company-level and System-level.

