From my understanding Steemit owns allot of rights such as the STEEM name. They also have a controling share of the token. Those things all now belong to TRON which more or less gives him control over STEEM.
For example, if he so choose to, the new owner could use his witness votes to appoint new top witnesses. I'm not saying that will happen but its a concern of some.
It's all up in the air at the moment, but I hope, like most smart business people that he leaves things as they are for the most part. Or at least introduces a lot more transparency, regular updates about what's going on, future developments and so forth.
From what I been reading, that's not the plan. It seems to me the plan is to do a token swap and move Steem(it) over to the TRON network.
More information should come out today.
As for powering down / buying / selling, no-one knows for sure.
Here is the medium announcement post.
https://medium.com/@TronFoundation/steemit-joining-tron-ecosystem-6ad66584d0b