See that wouldn't work tho, being able to transfer day 1 would completely mess with world firsts and everything. They would definitely need to put a timer on being able to transfer, most likely in the 1-2 month range.
But ya they definitely wont just make TBC on the same server, there are too many people that want classic to stay classic forever
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Kind of irrelevant how many stay, the hype day 1 again is going to far outweigh how much they have put into the project. This will be the same thing with TBC, day 1 hype and returning subs is what will pay for server upkeep for years.