There is no way to change the standard unit of measure once transactions have been posted to an item. All of the history tables contain quantities that reflect the standard unit of measure (they do not store a unit of measure). The sale and purchase UOM can be changed at any time because they would be used as a default during the next receipt or sale, and would be converted to the standard UOM when posted. It would actually make a lot of sense to disable the field once transactions have been posted. The argument for leaving it open is to allow a terminology change that wouldn't affect the actual quantity. Changing EACH to EA, or even BOX to CASE, as long as it is understood that they represent the same amount of the item.

Thanks for your reply that all makes sense, but I still need to change some items. So I need to adj the item to zero, and then receive it again at the new qty and unit price to get the item in at the new qty and unit price? Is there anything that needs to be run after adjusting it to zero?

There shouldn't be anything else you need to do. Adjust the stock out at the old UOM, change the UOM, and then adjust it back in at the new UOM. When looking at history, the original receipts and adjustments will look strange because they will just be numbers that seem to all be the new UOM. They will, however, balance out. Everything going forward will match the new UOM.