Hi RobynM1,
Yes you can tick the I Inventory option, you can even select an account and change the COG account. However when you click OK to save the changes, this is when MYOB 'may' tell you with a large red popup that you have sales or purchases on file for this item.
If you are able to save the changes, then yes it will work. Manually one by one or via import. Try to change via import, and ones that can be changed will be, ones that cannot be won't be. This is where you will need someone like MYOX Services.
Regards
Gavin