How will the leveling process with Cataclysm be changed?

I doubt it. I haven't heard that the way you level is changing, just the ways you can do so. Several of the zones are going to be completely redesigned, which is really exciting to me (like the Barrens will be split in half, a low level area and a higher level area).

WoW is and always will be a game friendly to people who want to solo, so I don't think anything is going to change in that respect.

Hello,

If you are refering to the "Kill monsters, do quests and gain experience" leveling process, that won't change.

Although what is going to change is the world of Azeroth, the world will be remodeled due to the cataclysm, zones and level range will also change, as well as quests. So there is going to be an excuse to replay the game and go through the "new" content of the earlier levels. Most people have called it a "facelift" for old vanilla WoW because it is going to remake all the cities (potentionally) and the zones to a better graphics version and help progress the storyline back to the world of Azeroth beside to make it external to other zones such as Northrend and Outlands.

You will also be able to flying in the new Azeroth.

So I guess this is what you can expect: New Zone and quests for Lv.1-60, Outlands and Northrend will stay the same then back to Azeroth for Lv.80-85 in particular involving zones with upgraded places to see.

Never know, we might actually get to know what's in Grim Bastol in Wetlands and that place in the south of Tanaris and of course the Maelstorm in the middle of the sea.

Yeah I know, can't wait for that either!

-D