Here's how EOS is able to be launched as an independant blockchain while retaining the tokens everyone has purchased.
Once development is completed, EOS will be marked as open source and published for anyone to publish and play with. Block.One has said they will not launch the EOS themselves which may sound confusing, however by nature blockchains are open source distributed systems.
EOS will be published as open source and the community is then free to copy and launch multiple EOS chains. All of them will be based on the same genesis block released by Block One. This means that people who had purchased EOS tokens, will be able to access them on all EOS chains that are started.
Over time only one of the chains will get the most support, and that's the one that block producers will produce blocks on, because the most popular chain will have the most value and block producers will support the copy the earn the most from. The EOS chain with the most user support will naturally become the official one, and this will become known as the official EOS.
This behaviour of open source blockchain ensures that no one will use a copy that isn't fairly distributed, or that has been tampered with. The community will naturally support the version with the most support and all tokens purchased today will be translated over to the live EOS blockchain.