I don't know much about the technical details, but what I've heard is a wholesale substitution or incorporation of BTS into the EOS blockchain, but either would undoubtedly come with at least a 1:1 exchange. EOS is still in "testing" - I would say development - so things are still very speculative. The idea is that if EOS is better, then why not switch? Or better yet, why not just take over EOS and continue calling yourself BitShares? FAQ 21 explains how that is a possibility. https://www.eos.io/faq.html