There are a few write-ups about this but the bottom line is that Windows NT, 2000, XP and for the forseeable future of Windows, Microsoft has not provided any indication that there is a supported "this will work" method for what you propose.
The only supported method is to backup what you can't lose and start over, then restore from backup.
I have a PC with a partitioned hard drive that is currently set to dual-boot Windows ME and Windows 2000 Pro. The C: drive is the ME drive, while Win2K is on the D: drive. How do I safely remove Windows ME and return the free space to the Windows 2000 partition? Can I then rename the drive back to C: ?