I think you're missing what I think is the most likely scenario from having closely read the announcement.

It sounds to me like they will make a new token on the Tron blockchain called "STEEM". They will then implement the Steem "Proof of Brain" system as a Tron smart contract that uses that new token and steemit.com and any other dapps that migrate over will switch to using that.

Then they tell the exchanges (with which they now own all of the relationships that Steemit, Inc had, on top of their existing relationships) that "STEEM" is now the Tron token version. My guess is Justin probably already discussed this with most major exchanges that list STEEM before he made the acquisition.

Then they have steemit.com, some other dapps, and all of the major exchanges considering the Tron token STEEM as the real STEEM, and just like that Steem is now on Tron.

The blockchain we know as Steem will still continue to run, but it will have lost its exchange listings, main dapp, and therefore most of its value. We will have to start all over to get exchanges to list it again and they will do so under another name like "Steem Classic" for example.

This is by far the most likely scenario to me. It makes perfect sense from Justin/Tron's perspective and I think the wording in the announcement alludes to it. It doesn't require them to do anything with the steemit stake, or bother with the witnesses on the current Steem chain.

A lot of people are focusing on the "token swap" term, but my guess is that was just a misnomer and it be more like what we know as an airdrop where for every STEEM/SP you have on the current Steem chain you get the equivalent of the Tron version.