I upgraded to Stretch following the normal procedure, changed the repositories, etc. But after all, I'm still running Chromium 56, a older version of the browser.
When I try to update the system, it checks both repositories, Jessie and stretch, I don't know why because I replaced Jessie with stretch source.list.
Now, no new version of Chromium appears when I run
sudo apt upgrade... maybe the repository, maybe something else... I don't know.
By the way, I have a medium-high end PC, but still prefer the silent browsing of my Raspberry Pi 3... found it so exciting.