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Read and test your way through the functions. Do so only on a copy of your spreadsheets and be sure that your test does not in anyway have any means to touch live data.
Then consider using the spreadsheet data as some RO (Read Only) Back-side data tables that are accessible by a Front-side database that performs the required lookups. Once the data is found the database side pastes the results into a new spreadsheet or table.
Plan it out and work step by step. Again - ensure that real data is safe.
Allows you to delete the test environment if things go astray so you can start over again if necessary.