altering rhythm

hi. when i try to write music, i have to figure out the pitches first, and /then/ i work on notating it with the correct rhythms. what i want to do is write (for example) a line in only quarter notes, and then adjust all the notes i've written to have the correct duration.

so things i want to be able to do, but can't figure out how:
- for example, i might decide that most of these notes will be eighth notes. i want to highlight all the quarter notes i've written down, then change them all to eighth notes.
- i might then want to individually change some of these notes (such as making some into, for example, sixteenth - dotted-eighth rhythms)

i don't know how to do this, though. when i try to highlight a section, and then press the eighth note button, nothing happens. and if i try to (for example) change two eighth notes into a sixteenth - dotted-eight rhtyhm, by selecting the first eight note and changing it into a sixteenth note, i get a sixteenth note, a sixteenth rest, and an eighth note. i then i have to highlight the sixteenth rest and re-enter the following pitch. (this is pretty laborious!)

does anyone know how to do what i want to do (ie write pitches first, then add rhythm)?