Hi MK
Technically speaking, you can change the decimal places rounding of an unit. Creating a new unit as a copy may be very painful, espcially if there are materials and master data already using this unit. It's a very complex procedure to change the unit of an existing material.
Also, it is very difficult to measure the impact of such change, since the unit of measure is used in almost every component/application area and there are thousands of programs using units of measures.
The only way to validate the change is to make it on a test system and ask your users to check if the process is correct, paying especial attention to the unit rounding and quantities.
BR
Caetano