change value of beginning notes in a measure

I'm writing chant in single measures per system. I have a measure with 19 beats, filled with all the notes and lyrics. I now realize that the first two notes are incorrect. They are an 1/8 note and a dotted quarter. I need to make them a quarter note and a half note, resp. So, I changed the measure to 20 beats, deleted the rest that it put at the end of the measure, and tried to change the value of the first two notes to a quarter note and half note. But in doing so, it deleted the next two notes after that. It doesn't allow me to simply change these values, even though there now are enough beats in the measure to do so. How then do I change these note values without having to redo the whole measure, and without it stealing away from the notes that follow after the ones I want to change? Thanks.