The Flow team is going to work in a few weeks on automatically archiving talk pages, so that we can enable Flow on pages where there are already existing conversations. Basically, this means moving the old discussions on an archive page, and leaving a link for "See archived talk page" visible on the new Flow board.
That means there'll be a minute where a currently active discussion would get interrupted, and have to be restarted on the new Flow board. That will be a pain, but it would only be a one-time inconvenience during that transition moment.
The team's goal for LiquidThreads transition is essentially the same -- turning the existing conversations into a form that we can archive, and preserve for the future. If we tried to turn ongoing LQT discussions into ongoing Flow discussions, we'd actually be spending more development time on archiving the deprecated feature than we would spend on archiving wiki talk pages.
There's a few more big features for Flow that we're working on this summer; we'll have some new things to show off pretty soon.
Danny