Tadalist and Writeboard both say: `But don’t worry — any [x] you already created here at [y].com will continue to work`

Which is nice.

Of the above, the only thing I might take issue with is the stranding of legacy Basecamp customers. Why don't they want to migrate? What can 37signals do for them? Answering these questions is important, and if they are already working on this then I see no problems.