Hi PhantomPE
AR2023 may not allow you to do this for some items. Once an item has been used, this setting cannot be changed. If you want contact details for a MYOB specialist who can do this for you, send me a private message or email.
Otherwise what moderators have suggested in the past is to manually edit each Item by adding a prefix of ZZ to the Item Number and make the Item Inactive. Then create a new item with the original item number and will the I Inventory ticked. Tedious and you will end up with 18k items.
Regards
Gavin