Welcome to the forum.
You seem to have the situation well understood. From what I've seen on the old forum and watching a local friend do the conversion last year, don't bother changing the sand until the conversion is complete. And yes, vacuum what you can off the bottom as anything there will consume chlorine unless it's been oxidized already. You'll also need a lot of bleach. May check local pool stores for the 10 or 12% stuff in refillable containers. If you can, take some pics of the progress.
Good Luck and welcome to the world of Chlorine.
Al