Agree on the product model
Define a stable external ID or unique SKU and rules for variants. An individual SKU for a color or size must be mapped to the corresponding option or individual item - depending on the store model. Fix currency, weight units, decimal format, language and empty field behavior: clear value or keep old.
Continue with: KeyCRM stock sync: ownership, reservations and checks.
Test a small complex sample
Take items with multiple photos, options, promotion, zero stock and long name. Upload them to the copy and repeat the same import. The second run should update the records, not duplicate the catalog. Check availability of image files, category links, and price display on product pages and in the cart.
Continue with: E-commerce filters: which pages should be indexed?.
Count the changes before the big launch
The report should separately show created, updated, missed and erroneous records. An item that is not in the new file should not be automatically removed without an agreed rule. After the import, check the sample by SKU and check the URL: the name change should not subtly break the old links.
Can stock levels be imported from two systems?
Only with defined priority and exchange rules. Otherwise, the next file may overwrite the current stock with a stale value.