Is it like a message board/forum just moving to new software? I thinkI think what matters, both ethically and legally, is the contract not the software.
Yahoo's TOS constitutes a contract between Yahoo and the members regarding the content they post. That contract may be transferable to successors or assigns (in the event of a sale of Yahoo's assets), but there's no way that it transfers to a competing service.
A message board/forum owner who changes hosting services may not need any permission from the members. In that case the members' contract is likely with the board/forum owner, and the owner's contract with the hosting service is a separate matter.
The distinction matters ethically because the member has a reasonable expectation of certain rights and obligations based on the terms of service that the member agreed to.