There are a couple ways they can do it. If it was me i would do this.
1. All current servers are upgraded to TBC. This is so players can continue in the communities they are apart of and have built up into the next expansion. This would also be as close to the original experience as you could get going into TBC.
2. create however many evergreen classic vanilla servers are needed and allow free transfers to those servers for a limited time. My guess is they wouldn't need many. Once people are finished with Naxx i don't see them hanging around on a mature server for long.
3. they could also have a couple "fresh" vanilla servers for people who want to start over and do it all again. its a popular thing for private servers. They could refresh these every couple years.
I think number one will almost assuredly happen. 2 and 3 could happen if Blizzard thinks there is enough demand for it.