I have a melody with lyrics. I want to keep pitches and lyrics and only change the rhythm, and the way I have been trying is to activate insert mode, select the first note of the melody, change note value with a number key, arrow to the next note, etc. But the lyrics will not stay.
Is there a method of doing this? Tell me that there is a “lock to pitch and preserve lyrics”-button hidden somewhere… 
You should find that if you use the “lock pitches” tool (shortcut L during note input) that you can happily repitch the notes without disturbing the lyrics.
Yes, that I understood, but can I do the opposite? Happily “rerhythmicize” without disturbing the lyrics?
Oh, I see what you mean. No, I’m afraid that’s not as straightforward at the moment, but it is something that we are planning to address.
It depends on how intricate is the change you’re looking for, but I’d say Insert mode deals with most situations already. Just select a note or group of notes and change their duration with Insert toggled on — no pitch will be deleted.