Unless that's his plan, I don't think that he'll remove Alderney as it'll cause problems with the storyline and other gameplay stuff, pretty sure this is just an add-on, not a replacement.
Liberty City and Alderney, well, the entire original map should be left intact because of many reasons, one being the multiplayer(i really don't want an another IV:SA type of failure in multiplayer) and the other is that it's impractical to upload huge original files from the game itself that Rockstar Games have created.
Anyway, i've made a map idea schematic...
It's very hard and in some cases impossible to be totally correct geographically, not to mention in capacity of the map due to the given space that i'm working with(there's not a lot of free space to build new maps with Liberty City being in the center of the game's world!), but i think this would be fine enough for the game anyway...
Here, take a look at my idea for the map:
Stroked borders indicate the approximate space that it would take up in the game.
Yes, Carcer City is way too small, but there's no other way to fit everything in the same map and i'm just 1 guy after all, so working on a HD city with a huge size that it's supposed to have is just a fiction of an achievement for me.
That's why it would be nice if there were some more knowledgeable people interested in this map to help me out in building it