Yes, I raided BC Kara to Sunwell. While I’d love to finish Sunwell pre-nerf as my BC guild stopped raiding on Brutallus when it was nerfed I didn’t see much of Vanilla raiding content until BC and later. I learned to play my druid and tank/dps in BC.
I’m sure there’s a good portion that would change to TBC servers and we’d get an influx of TBC pserver players and probably some Wrath holdouts. However, I don’t know if it would be as big as people think. I loved BC, but way too many have nostalgia for Wrath. Wrath was mediocre (story line was only truly amazing part…and Ulduar) in comparison to me, but there’s a very large player base for that expansion.
I doubt we’ll see hints of BC until Naxx has been out for a while. Blizzard basically negated Naxx in Vanilla with the announcement of TBC launch and the hype train that it brought. Guilds quit raiding. People love that 1% stat, but ignore the surrounding issues raiding guild had due to TBC hype. At the time it was “why raid when all the upcoming gear may just replace it all?”. And I don’t think they want to relive that again.