In most applications, you can save an open file under a new file name with File > Save as... or shift-command-S.
In recent versions of Pages and TextEdit, the command is File > Duplicate > File (again!) > Save.
There is no keyboard shortcut for this, unless you customise your keyboard shortcuts in System Preferences > Keyboard > Keyboard Shortcuts > Application Shortcuts.
This admittedly makes Save as... more cumbersome to use, but it is a (somewhat) logical consequence of the Autosave feature of Pages, which is so automatic that you never should have to save a file again. Almost.
In recent versions of Pages and TextEdit, the command is File > Duplicate > File (again!) > Save.
There is no keyboard shortcut for this, unless you customise your keyboard shortcuts in System Preferences > Keyboard > Keyboard Shortcuts > Application Shortcuts.
This admittedly makes Save as... more cumbersome to use, but it is a (somewhat) logical consequence of the Autosave feature of Pages, which is so automatic that you never should have to save a file again. Almost.