You can change the stock-tracking method while the product is a draft. Open the draft from Drafts, change the kind or the Inventory mode in Step 2, and save it.
After publishing, the form shows Inventory settings locked after creation, and the method can no longer be changed. Quantities are never typed in the form after the first save: open Inventory and press Adjust stock on the product's line, or use the Adjust stock link in Step 2 of the form to open the product's own stock page. For per-variant stock, each size or colour has its own line in Inventory.
If a published product needs a different tracking method, create a new product with the right settings.