I checked this mor thourougly than I usually would check articles from @404mediaco because I had seen several outlets mention the 340 million number, and very few mention the 400m. So I was curious about which was right. I followed 404's link to PIRG and then their link to Forbes. There I read this:
"Of the 700 million Windows 10 users, it’s thought more than 400 million can upgrade, with at least 240 million PCs missing that security hardware hurdle."
_can_ upgrade it says for the 400 number!