Relabel levels in pandas

In a pandas DataFrame I'm trying to relabel the two levels of a variable with one single name but leave the 'Nan' values in the variable untouched.

Below is a reproducible example using a modified version of the 'mtcars' dataset. Here I want to relabel the 'yes' and 'no' levels of the 'am' variable to 'new' for example.

                    mpg   cyl  disp  hp drat    wt  qsec vs  am  
Mazda RX4           21.0  six 160.0 110 3.90 2.620 16.46  0  yes     
Mazda RX4 Wag       21.0  two 160.0 110 3.90 2.875 17.02  0  NaN    
Datsun 710          22.8  six 108.0  93 3.85 2.320 18.61  1  no    
Hornet 4 Drive      21.4  two 258.0 110 3.08 3.215 19.44  1  NaN   
Hornet Sportabout   18.7  six 360.0 175 3.15 3.440 17.02  0  yes  
Valiant             18.1  two 225.0 105 2.76 3.460 20.22  1  NaN   
Duster 360          14.3  two 360.0 245 3.21 3.570 15.84  0  no   

Result would look like this:

                    mpg   cyl  disp  hp drat    wt  qsec vs  am  
Mazda RX4           21.0  six 160.0 110 3.90 2.620 16.46  0  new     
Mazda RX4 Wag       21.0  two 160.0 110 3.90 2.875 17.02  0  NaN    
Datsun 710          22.8  six 108.0  93 3.85 2.320 18.61  1  new    
Hornet 4 Drive      21.4  two 258.0 110 3.08 3.215 19.44  1  NaN   
Hornet Sportabout   18.7  six 360.0 175 3.15 3.440 17.02  0  new  
Valiant             18.1  two 225.0 105 2.76 3.460 20.22  1  NaN   
Duster 360          14.3  two 360.0 245 3.21 3.570 15.84  0  new