Dividing (or multiplying) note lengths?

[Hilary Snaden]

I'm typesetting a Palestrina motet, possibly the first of several. For
ease of proofreading, I've used the original (long) note values from the
collected works edition, but I'd like to generate output with halved
note-values, as is current practice. Can Lilypond do this automagically,
or must I explicitly change the note lengths in the input file? (If the
latter, I'll probably write a Perl or Python script to process the
initial input files.)

--
Hilary Snaden