Hi Francesco,
This is strange. I have never seen editable HOST NAME. HOST NAME is picked from the CS for which the CA is created. Ideally you should be using the same HOST NAME that has been defined in the CS.
Service URL is generated by the combination of all the values mentioned in the above fields - Protocol, Host Name (which is picked from CS), Port and Path. So this need not be editable.
So if you want to make use the different HOST NAME you will have to make the changes in CS and activate it. Once you do this make sure you Save and Reactivate the CA. (We have experienced sometimes Changes do not reflects in CA until you Save and Reactivate them after Saving the changes in CS)
Hope this helps.
Chandan