It's a bad idea to rename those folders. The operating system and other Apple software expect them to be named a certain way. Like renaming your Applications folder or moving certain items from it, renaming Music, Movies, and Pictures is a bad idea.
You could copy them to an external drive, just drag-drop, but I wouldn't move them wholesale. It'd be safer to leave them in place but move the data, libraries, or files inside them for use on an external drive, if that's what you're going for. It's also very easy to put iTunes and iPhoto libraries on external drives without needing to tinker with Music, Movies, or Pictures at all.