Just a quick question, for Broonie and anyone else who knows the answer:

Are chest contents going to be completely preserved when visiting the old world via portal, or are we going to get another chest transfer like the last one?

Or perhaps we'll all have to start with nothing?

The world will be preserved, accessed through a portal. Nothing will change.

Does that mean that the current end, skyhigh, and waterworlds are going to be preserved as well, or are we going to get new ones?

nether and the end will be completly reset i assume. So if you have anything stored there you should move it. Nice thing about that: new dragon fight!
skyhigh and waterworld already are custom worlds and will remain unchanged.

Disclaimer: i'm not Broonie and do not actually know. This is just my guess as to how it will happen.

Nether will be reset; along the end.

Waterworld and others will not be reset unless upgrades to the generator are applied.