I don't think upgrading is going to solve the problem. The 502 Bad Gateway error appears to be one of Nginx configuration ... and/or php-fpm. That Google search for that error showed links to various forums, etc. and if you looked at them, even if not about moodle, all of them were discussions about Nginx config.
Concentrate on config of nginx and php-fpm. Check the error logs of nginx.
Like I said, don't run nginx/php-fpm so have never seen a 502 Bad gateway error. Not saying you need to switch to something else ... but will say tnat I tinkered with nginx/php-fpm this weekend on a tinker box. That combo does use or could use a proxy config ... right on the server itself. Didn't get to the point of installing a Moodle on the tinker box ... didn't really need to ... 8 Gig Rackspace box with only the DB's for Moodle installed and tweaked via Tuner didn't gain the significant memory I was seeking.
Do know that nginx is harder to configure and appears to require definitions of a lot of blocks ... even for little things. No htaccess and some other minor things.
Maybe an Nginx expert will pop in here and help!!! ;)
'spirit of sharing', Ken