How to replace null values in attribute table ArcGIS 10.2

I have a string field 'Oneway' with three values '+' '-' and Null showing a street accessibility, I would like to replace them with values 'F' 'T' and null as the script for Network dataset expect these values.However it seems that it is not possible to replace values if Null values are present. I tried also changing the field type to double or text but the field properties are not editable somehow (I tried this in arc catalog). I am only able to replace value '+' and '-' with numbers using the field calculator but need to replace them with letters.