If you have multiple CD* files ie CD1, CD2 CD3...and you want to make this substitution for all instances (which I guess you do from your question)... then you should use a for loop. you can do this from a script or a command line. I'd suggest script so that you can run it again...or modify it for later use.
for files in /u1/walter/CD*
sed -e 's/D/walter/g' $files > temp_file
mv temp_file $files
This will take each file in the /u1/walter directory that starts with CD make the substitution to the file and direct the output to a file called temp_file. It will then rename the temp_file to be the origianl filename.
To allow execution change the permissions on the file using chmod 755 ask. Then run as you have tried with ./ask
Hope this helps. (the -e syntax is probably optional in the sed command if you are only using one substitution).