2nd Generation Apple TV to get UI/1080p update?
Pretty sure that there are hardware differences allowing support for 1080p, not present in prior gen Apple TVs. So if that is the case, then a software update can't add in that support. We will not know for sure until we can see the spec and a complete teardown of the new Apple TV. We might be in luck for a UI update, though.
So according to ars techinca's Hands-on and questions answered: third-gen iPad, third-gen Apple TV

Apple TV
First let's start with the Apple TV. The device looks the same as the second-generation device and, in fact, the new interface that Apple touted during the event will also be coming to the second-generation version via software update. The only thing new about the third-gen Apple TV is 1080p capabilities plus a more direct link into iCloud for photo syncing. If all you want is the new and improved interface (which I do like, as an Apple TV fan), you'll be able to get it from Apple on your second-gen device without having to pay anything.

And as for hardware changes that's a yes to according to the same article as well.

We asked whether the third-gen Apple TV had the same A5X processor as that in the new iPad (as was previously rumored), but an Apple spokesperson told us that it has a &quot;single-core version of the A5&quot; and it's not the same as what's in the iPad. The CPU is capable of handling 1080p HD video.