What the QB people are saying is very accurate, if perhaps grossly understated.  IF YOU CHANGE AN ITEM FROM NON-INVENTORY to INVENTORY, QB will go back to the beginning of its time of installation, and change/pervert EVERY SINGLE TRANSACTION that used this particular item.  It will change your GL and everything associated with those transactions.  If you then pull an old hardcopy report from, say, two years prior, it will NOT BE THE SAME AS THE one you ran right after you changed the inventory type. 

Functionally, the ability to change to an inventory item in QB is less than useless.  With the fur-ball that would result from doing so, you'd have to be nuts to do this.  QB should put an effectivity date on such changes, but this is not the kind of functionality that we'll ever see.  This is truly just another example of QB being a lightweight package that is trying, and failing, to hit above its weight class.  This is not a package that has any place in most truly industrial manufacturing environments.  If you need a real ERP system for manufacturing or even component assembly, you WILL need to spend more to get a viable product.