Resetting the DME isn't that big a deal, and maybe even good to 'reboot' it occaisionally, besides, it only takes a couple drive cycles for it to re-adapt - lots of unwarranted fear of doing this.
Don't believe that you need a code for an '03 radio. If so, you can always pull it from the unit before you pull the battery.
If all this scares you, you can always get a
memory keeper like this.
Cheers!