Change the value of a field depending on the selection of another I need to change the value of a field based on the selection of another.   I also don't want the user to have the ability to change this value.  I tried using #disabled = true, but the values don't save, and the same with #access. 

I'm not sure how to go about this?  #disabled seems to stop values from being saved - what alternatives are there?  

This surely can't be unique, I know often date fields do this.
Converting the element to a value type might be easiest:

// In a form alter hook.
$form['field_foo'][LANGUAGE_NONE][0]['value']['#type'] = 'value';
$form['field_foo'][LANGUAGE_NONE][0]['value']['#value'] = 'some static value';