Globally Change the Length of All Notes?

Is there a way to globally change the length of all notes in a score? For example, taking a piece that is currently written in 3/8 and converting it to 3/4 - I want the same three notes to be in the first measure, but would like them now to be quarter notes rather than eighth notes. Another example would be a piece of early music, where I have a piece written in 4/4 but I would like to "reformat" it into 4/2 (so each measure gets two whole notes, instead of two half notes).

Said another way, I'm looking for a function similar to "transpose" - instead of making a global change to the pitch of every note, I want to make a global change to the length of every note.

Any ideas? Thanks -