Hi CassG , if you cannot untick the "I inventory" option manually, then you will not be able to change it through the import/export process. (Once transactions have been recorded, that option cannot be changed).
Generally what people do is add a prefix such as "ZZ" to the relevant items, then create a new item without the "I inventory".
Re-creating items can be done by exporting the existing list (before adding a prefix), and removing the "I inventory" option by editing the text file in Excel, then save as text. However, adding the prefix also can only be done manually, the import/export wizard cannot be used to change item numbers.