i am pretty sure they said that they won’t be implementing tbc era servers so i would assume that all of your tbc characters will automatically become wotlk characters once pre patch hits
that is just my assumption based on what they said and they might reconsider it at a later date idk
If TBC Era realms could happen, once Wrath prepatch date hits, you would have to choose for each character on selection screen whenever you would like to play that character on TBC Era copy of the realm or continue playing in Wrath. Untill that choise character would be locked.
I was there during original Wrath prepatch and it is best prepatch event in WoW history. A bit chaotic, but extremely fun.
All the realms change from TBC to WotLK realms, that’s all, just like in 2008. They are releasing new realms during the Wrath pre-patch and you cannot transfer to them, only make new characters. They will all progress to Wrath just like the current realms will.