Can I just let my braids turn into dreads? - Dreadlocks FAQ's
You can start dreadlocks with braids but they will generally look pretty crappy (lots of loose hair and fuzz) and it takes a really long time for them to start looking like dreads at all. You can always see the pattern of the braid inside the dread since it's only the hair on the surface that's actually able to lock. As the hair slips out of the braids over time they get very fuzzy. Eventually enough hair comes out that some of it may start to dread.
It usually looks like you have the worst maintained braids in the world rather than dreadlocks. Probably not the look you're after. =]
If you do have your hair in braids and the sections are the right size for dreads you can unbraid and backcomb each section one at a time. This will save you having to section the hair again. This is a great way to have your sectioning done before your dread party. Any braid shop can quickly section and braid your hair with any size square sections you want. A rubber band at the end will keep the braid intact until you're ready to remove it and backcomb the hair into a dread.