Need to copy files to existing directory and remove files already there with the same name but different extension

Question

EDIT: Total rewrite of question for clarity.

I have a directory tree (new) with a bunch of files of with an extension of .new. I have an identical tree (old) where many of the files have names identical to those in the new tree except that the extension is .old. I would like to copy all of the .new files from the new directory tree into the old directory tree which contains the .old files. As a file with a .new extension is written into the old directory tree, I would like to delete any file with the same name but a .old extension.

So, if in the new directory tree, there is a file named new/foo/bar/file.new, it will be copied to the old directory tree as old/foo/bar/file.new and then the file old/foo/bar/file.old will be deleted if it exits.

EDIT #1

This answer was hashed out below (using the old question that had extraneous background information that was confusing). See the actual solution that I worked out below as one of the answers.

Answer 1

The basic approach would be to run this from the directory containing the mp3 files:

for i in *.mp3; do cp "$i" /path/to/flac/ && rm path/to/flac/"${i%.*}".flac; done

This goes through all files whose name ends in .mp3, copies each to /path/to/flac/ which should be the directory containing the flac files and if the copy was successful, deletes the corresponding .flac file. The trick is using the shell's string manipulation capabilities, ${i%.*} will remove the extension so rm ${i%.*}.flac will delete the file that has the same name as the current mp3 file but a .flac extension.

To illustrate:

$ foo='abc.def'
$ echo ${foo%.*}
abc

Answer 2

This was the final answer that got hashed out in the comments for terdons answer.

cd new
for i in */*/*.new; do cp "$i" "path/to/old/${i}" && rm "path/to/old/${i//new/old}"; done