At Sage 500 ERP, we HIGHLY discourage changing an item's type in Sage 500 ERP.  This is why I do not believe there is a UI capability to do this.  Doing so using other means is unsupported.

In general, we recommend adding new items with the correct item type, and replacing the previous item with the new item where it is used.  If there are only a few items involved, this tedious task may not be so bad, but it could possibly distort histories at a high level, among other things.  If the item name is really important, you would rename the old item to something similar and create the new item with the old item's names.  Renaming is just a cosmetic issue but may be important for other reasons (report descriptions, external integrations, etc.).  Internally, when "ItemA" gets renamed to "ItemB", the internal reference to the item stays the same so if "ItemA" was on a routing and gets renamed, the routing would show "ItemB" going forward.

Given that BIG warning, of any item type change finished good to or from Raw Material is likely the least risky of any other combination.  The actual process of making this happen is relatively easy but the complications and hidden side-effects may not be readily apparent.

If you wish to go this route I would suggest that you be sure to work with an experienced partner for their input and execution especially in the manufacturing module.  They may know of  the various risks and trade-offs involved in such an effort.  Some of them might even monitor these threads. 

I am not endorsing any type of solution here and just want to make sure that you understand it may not be as simple as it seems like it appears to be.

In my experience, the problem with making this change isn't so much of a problem with your BOMs or usage, it's the other item attributes and associated data that could be an issue. That could extend to the tracking and valuation methods (standard isn't typical for raw materials), the product line associations, item attributes, UOM, pricing, replenishment, customer or vendor item associations and aliasing, as well as pick and putaway configuration. Verify any attributes you might need to modify if you change the item type and how that will affect your BOMs and purchases or production of those materials.

I think changing the item type in SQL it is risky, but we did it about 8 years ago for about 600-800 items. It did work for us. It retained all sales history and such. Your case might be vary depending on what Sage modules you use.

did you update the timItem table directly?