As far as I understand this, to your browser and to Google, they're the same page (this really seems to only apply to the homepage, other pages "/" really matters a lot).
I would not worry about changing the canonical, in fact it's technically the 'right" location of the page. When it comes to the HTTP docs, a request to the root needs "/". All modern web browsers add it back in for you even if it's not there. This is I believe why the canonical is set as it is there in yoast.
I covered this in a blog as someone asked this at Pubcon this year in a keynote.
http://www.winstondigitalmarketing.com/yoast-homepage-canonical-correct-include-trailing-slash/
