It is not possible to change Unit & SalesUnit on parts that have transaction documents registered. This will create problems with the history regarding orders /stock and inventory figures. 
Changing the unit can be done by creating a new part using the option: F12: Copy Part.

The Unit can only be edited while the article is in status 1.
One can write-off inventory on the old part, add this to the new part if in stock. 
The old part should be closed (forward to ST:9).