When a file is digitally signed the Save As dialog will automatically appear. However, you should be able to overwrite the original file, if you wish to.
Thank you for your reply, there isn't a problem overwriting the original fortunately but it takes a long time to find it. I was hoping I could change something in the preferences to speed the process up.
For me, the Save As dialog opens to the same folder as the original file,
with its name pre-populated into the File Name field, so I just need to
press Enter twice to overwrite it...