The riskiest way to leave a billing spreadsheet is to pick a go-live date and switch everything on it. If the new system is even slightly wrong on one contract shape, you find out from an angry customer instead of from a reconciliation report.
The order that actually works starts before any contract is imported: get the catalog right. Customers, products, and list prices need to exist and be correct, because a contract that lands on a blank catalog either gets rejected or — worse — silently creates a duplicate customer. Sonic can materialise the customer and products at contract approval time rather than forcing you to pre-populate everything by hand, but the catalog still needs to be the source of truth before volume ramps.
Next, import contracts in cohorts, not all at once. Start with the renewal shape you understand best — a flat monthly fee with no usage component is a better first cohort than a contract with three deferred tranches and a seat minimum. Each contract's structured result is reviewable before it becomes a live schedule; use that review step deliberately for the first cohort, because it is where you calibrate trust in the extraction.
Then run in parallel for at least one full billing cycle per cohort. Let the spreadsheet keep issuing the real invoices. Let Sonic compute what it would have billed, and compare the two line by line. Disagreements are not failures — they are almost always a contract term the spreadsheet was already getting wrong, or a term Sonic needs configured differently. Either way, you want to find that out on a comparison report, not on a customer's desk.
Only when a cohort's numbers agree for a full cycle should that cohort's invoices start coming from Sonic. The spreadsheet does not need a funeral; it shrinks cohort by cohort until the last renewal moves over. That is slower than a weekend migration. It is also the version where nobody discovers the new system was wrong by re-billing a customer.