When upgrading expansions, you do not need to reinstall the game client. You just have to upgrade your account from either the online upgrade or the authorization key that's on the CD sleeve in the retail box copies from the store. They do the same thing. The expansions must be upgraded in the following order - you CANNOT skip expansions.
Note: The Burning Crusade and Wrath of the Lich King are now included in the battlechest! I'm still separating their content sections in this listing though.
BC = Burning Crusade
WotLK = Wrath of the Lich King
Cata = Cataclysm
Mists = Mists of Pandaria
Battlechest = Classic + BC + WotLK in one package!
You can purchase all of the expansions up through Mists for $59.99 on the Blizzard store!
You don't need BC unless: (( included in the base game now ))
-you want to level in Outland at level 58
-you want to level past 60
-you want to level professions past 300
-you want to play in the Eye of the Storm battleground
-you want to do level 70 raids/instances
You don't need WotLK unless: (( included in the base game now ))
-you want to level in Northrend at level 68
-you want to play past level 70
-you want to level professions past 375
-you want to play a Death Knight at level 55
-you want to play in the Strand of the Ancients, Isle of Conquest, or Wintergrasp battlegrounds
-you want to do level 80 raids/instances
You don't need Cata unless:
-you want to play past level 80
-you want to level professions past 450
-you want to play in the Twin Peaks, Battle for Gilneas, or Tol Barad battlegrounds
-you want to do level 85 raids/instances
-you want to do level 85 arena fights
-you want to fly in Azeroth at level 60
-you want to level Archeology at level 20
You don't need Mists unless:
-you want to play past level 85
-you want to level professions past 525
-you want to play in the Silvershard Mines or Temple of Kotmogu battlegrounds
-you want to do level 90 raids/instances
-you want to do level 90 arena fights
-you want to play a Monk class
---------- Starter/Trial Accounts ----------
Blizzard FAQ including restrictions:
The Blizzard FAQ is wrong in several places.
- It is not Free to Play because it's just a trial account without a time limit.
- You can roll all races of characters (including Pandaren when Mists launches).
- At level 20 for PvP you're put into the exp-locked battlegrounds.
- Secondary professions are NOT capped at 100.
- Only players near your level can see what you type in /say chat (up to level 25-ish we think; Blizzard hasn't actually told us).
- You cannot create custom emotes (/e or /me or /em in chat).
- You can group with players of any level, but you cannot invite anyone to a group. A full account must invite you to the group.
---------- General FAQ's ----------
Q: Do I need Cataclysm to see the revamped 1-60 content?
A: No. That was included in a patch and all players are playing within the same game world.
Q: How can I pay for WoW? When do I have to pay?
A: Once you upgrade to a full account and your free 30 days run out, you have to start paying the subscription either via credit card or by adding gamecards to your account. Whether or not you have expansions doesn't change the monthly price, but you can get game time for a couple bucks less per month if you buy it in 3- or 6-month packages. More details can be found here:
Q: Can I make a worgen, goblin, or Pandaren regardless of my account type?
A: Yes! Races are no longer restricted to expansions. Classes like the Monk and Death Knight are still restricted.