Tested on both subject and talk pages. On the page before the conversion, I put a small amount of text (no sections).
After conversion, it leaves an empty Flow board (it should copy over the header). The history is completely broken:
Failed to load history content.
Return to Main Page.
[dbe2861d] /w/index.php?title=Wiki_talk:La_taverna_2015-05-15_retest_no_subpage&action=history Flow\Exception\InvalidDataException from line 55 of /vagrant/mediawiki/extensions/Flow/includes/Data/Pager/HistoryPager.php: Unable to load history for shdj1m9dbbi7izb6
Backtrace:
#0 /vagrant/mediawiki/extensions/Flow/includes/Block/BoardHistory.php(61): Flow\Data\Pager\HistoryPager->doQuery()
#1 /vagrant/mediawiki/extensions/Flow/includes/View.php(170): Flow\Block\BoardHistoryBlock->renderApi(array)
#2 /vagrant/mediawiki/extensions/Flow/includes/View.php(70): Flow\View->buildApiResponse(Flow\WorkflowLoader, array, string, array)
#3 /vagrant/mediawiki/extensions/Flow/includes/Actions/Action.php(101): Flow\View->show(Flow\WorkflowLoader, string)
#4 /vagrant/mediawiki/extensions/Flow/includes/Actions/Action.php(46): Flow\Actions\FlowAction->showForAction(string)
#5 /vagrant/mediawiki/includes/MediaWiki.php(413): Flow\Actions\FlowAction->show()
#6 /vagrant/mediawiki/includes/MediaWiki.php(291): MediaWiki->performAction(Article, Title)
#7 /vagrant/mediawiki/includes/MediaWiki.php(583): MediaWiki->performRequest()
#8 /vagrant/mediawiki/includes/MediaWiki.php(432): MediaWiki->main()
#9 /vagrant/mediawiki/index.php(46): MediaWiki->run()
#10 /var/www/w/index.php(5): include(string)
#11 {main}