converting notes to triples

I've converted a PDF to a musescore file, and now I need to hand-correct a few things.
The measure shown here should contain 4 sets of triplets. Instead, it was coverted as 2 sets of triples, followed by two sets of non-triples.
Can I convert them by hand without deleting and re-entering them?
I can't figure out any key combination. If select one of the triples (ie. the three notes), and press CMD-3, then each note is converted to a triplet so I have note-rest-rest, note-rest-rest, note-rest-rest. And if I select the notes and then press the quarter note in the control panel, then the 3 notes are deleted and replaced with a quarter note.