ok the code looks strange like this, let me clearify it:

restart: with(Units[Standard]): Units[UseSystem]('SI'):

T:=1200*Unit('degF');

                          1200 Units:-Unit('degF')

convert(T,'units','K');

                            2000                 
                             ---- Units:-Unit('K')

                              3                   

evalf(%);                  

                  666.6666667 Units:-Unit('K')