Don't make the mistake of doing it. You may think that WOW TBC Gold you will continue to play your character Classic however, you will not. This is especially applicable if you've gone through Naxxramas. If you want to continue playing Vanilla, just start over with a different character, it's going to be significantly more enjoyable and rewarding
What is troubling is those who are looking to complete max, but don't need to do it due to changes to the tbc system or stay the old school and find new guilds and start playing nax raids from scratch by creating completely new raids. They will need to purchase a service that will duplicate them in order to start tbc. Because the original wow community is no longer in existence, the cloning service should be completely free.
All servers currently running will be TBC servers. New Vanilla versions of these servers will be made available. You can choose to activating the character either the Vanilla or TBC version. It is necessary to pay if you wish to activate the same character on both.
EDIT: From Blizz's own blog post on the topic. On patch day, players will be able to launch Battle.net desktop and select the game you would like to play: WoW Classic or Burning Crusade Classic. Your current characters will appear on both types of games under your old name or. After selecting a game select the character you want to play and then confirm. If you decide that you'd like to play your character in both types of games then you can avail a paid option to access the cloned character for both game types. All you need to do is login like a normal user and then play.
Since the copy has been completed before release, yes. Classic includes all the characters, therefore there is no delay when you choose Classic. TBC can cause delays, due to the fact that it isn't immediate. It can also result in a lot more characters being copied at the same time. This could result in the system becoming unresponsive.
The boost will usually become problematic because bots most likely utilize it, creating even more issues with bots. However, if bots were properly removed, I don't think the boost would cause any issue. My friends and I all go back to classic because we enjoy TBC. The reason they join is that they can increase beyond vanilla content leveling.
All our characters exist in Classic already. They don't really want down time for people to go to TBC. The entire database will be copied prior to launch. If you select TBC, they will change the "active" portion of the buy WOW Classic Burning Crusade Gold TBC database to true and flip the "active" bit on the Classic database will be changed to false. Then you can be charged $35 to flip the Classic bit back.