How can I be downgraded to a lower league?

Your hidden MMR (ELO-like rating) will plummet below a diamond threshold even while you remain in Diamond. After just a few (~10) games losing against "evenly matched" opponents, you should start playing games where you are considered "favored", and losing them. Eventually you could theoretically be always matched with bronze, even though your publicly visible league does not necessarily need to match your MMR.

However, I have noticed that bnet seems to only stray so far from your league. I could be wrong about that one, but I know a gold-level friend of mine that, after 25 losses in a row, was still being matched with silver and gold players. They could have rules in place that you can't be matched with someone more than 1 league away from your own.

All this is beside the point at the moment. With the current ladder lock, you will not be demoted or promoted no matter how many losses you incur.

tl;dr: You'll have to wait for the placement match in Season 3.