Could someone with domain knowledge put this development into context for me?
If you don't cash out, then they are claiming that your account will be transferred to the new site with balances intact, and withdrawals will be available through the new site with the remaining 45% tied up in a "debt token". The rough idea is that these tokens will be able to be redeemed in the future for some amount.
The calculus for account-holders is such:
1) If you choose to withdraw 55% now, that's the maximum you'll ever get (also, no one has even gotten anything back yet as they say it takes up to 72 hours). So you take the 45% haircut as cost of leaving your coins on an exchange instead of in a wallet you control.
2) If you wait, there's no telling what will happen. The happy path is you wait, then cash out your 55%, and then the 45% is eventually paid back to you over time. This path does have precedence. The unhappy paths are plentiful. The site could get shut down again, for good. They could bar you from withdrawing without proper verification. This could mean providing sensitive items such as your passport and address. Not great from an anonymity standpoint. Then who knows what the FBI or IRS would do.
I lost 100% of what I had in Mt Gox and it took a long time to become ok with that. To get 55% of your stuff back is a rare opportunity.
Who knows, you could get 10 cents on the dollar for those tokens.
Has a bit of an air of "you won't do as we tell you? watch how we fuck your shit up"