To move a Unit from one customer to another 

Go to Customer Files

[Ctrl/U] Units 

[F8] Move

Note: make a note on the unit to record who previously owned it as some searches for history  (by customer and unit) may be limited if the former owner is not known.  History searches by only the Vin number are always the most accurate if unit numbers change or accounts change.  

To change the Unit Number

Go to Customer Files

[Ctrl/U] Units

[F4] Edit

edit the Unit number and press Enter or Tab to leave the field, the system will prompt "do you want to move / update unit history"...  Select Yes 

Note:  make a note of the unit number change for future information