It appears that changing a capital letter package name like:
MONITOR into monitor
Is impossible in eclipse helios version. This happen to me on Windows 7 64bit.
Solution: Change this "MONITOR" into something like this "MONITORa" as a temporary name.
Then change the temporary name to "monitor" this should work.
In case you are using svn or subclipse plugin for eclipse, i thinks that you will need to do commit between changes.