Changing upper/lower case strings

[Mario Dona]

Does anyone know if vi can change a word from uppercase to lowercase
(or vice-versa) with one command?  I know that placing the cursor under
a character and typing the tilde (~) changes the character's case, but
I can't figure out a way of doing that with a word.  I suppose there
must be a way of applying that command to a string that is delimited by
a space character.  Any help would be appreciated.