Being a CD that "Burns" the image to the disk, any changes made to the file cannot be re-written to the existing file on the disk therefore, the files are "Read only" and cannot be ammended.
CD/RW can rewrite changes to the file as the CD is able to have images burned to it several times. However, they may still be "Read only" by default.
In short, your data is perfectly safe, you will simply be writing a file back to your hard drive but it may still have the read only attribue which you will need to remove before you can ammend it. If it is simply a file that you wont be working on you will not even need to do this.