Copy and Rename Multiple Files with Regular Expressions in bash

Here is a solution which use the find command:

find . -name '20*' | while read oldname; do echo mv "$oldname" "${oldname/20/10}"; done

This command does not actually do your bidding, it only prints out what should be done. Review the output and if you are happy, remove the echo command and run it for real.

Just wanna add to Explosion Pill's answer. On OS X though, you must say

mv "${file}" "${file_expression}"

Or the mv command does not recognize it.

You just have a little bit of incorrect syntax is all:

for file in */20?????.*; do mv $file ${file/20/10}; done

1. Remove quotes from the argument to in. Otherwise, the filename expansion does not occur.
2. The $ in the substitution should go before the bracket