I would probably start with a straight file search in the source code directory. I'm on a Mac so would do this to give me a list of source files that instantiate new HashMaps.
find . -name "*.java" -exec grep -H "new HashMap" {} \;
Then go through the list of source files and replace the HashMaps.
Or, you could use the search and replace functionality of your
IDE, or
sed to replace "new HashMap" with "new ConcurrentHashMap". Then you just chase down the compile errors to fix the missing import statements, which is a pretty trivial thing to do if you have a decent IDE.