Previous discussions are archived and a link is created on the Flow board description (right side on LTR wikis). Was it the case? On which wiki have you enabled Flow?
Copying existing discussions is very, very complicated to do: it require to parse an entire unstructured wiki page and find every element. It is quite trivial for titles, bot not when you have to attribute authors to every sentence, when big differences about indenting or signing exist across the wikis. When I've requested the conversion of my volunteer user talk page to Flow, I've did it when I had a few ongoing discussions. Other users where also redirected to the archive when they had notifications of new messages on old conversations on their watchlist.