I was updating from the 16gig bootstrap to the current (21gig) torrent and noticed they don't go side by side initially (ie. the 21 gig one doesn't start off from where the 16gig one stopped). I deleted both torrents (not the actual bootstrap file) and loaded the 21 gig torrent by itself, it (uTorrent that is) started checking the file as I would expect, but then started downloading from 0%. I expected it would start downloading right around 70% or so. Is this expectation wrong? Or did I do it wrong?

TLDR; How do I update the torrent to a more recent version without downloading from scratch?