You could do an export of all item record data, then re import as inventory items. Then inactivate the old non inventory part
However it would depend on how many open transactions and active BOMs are effected by these items. That is where the real hard work comes in. Updating transactions and BOMs to use the new inventory item instead of the non inventory item is very time consuming and tedious
I don't know of any โfastโ way to do that