Heroku: upgrading Ruby application from Cedar-14 to Heroku-18

I am working on upgrading the heroku stack of my Ruby application from Cedar-18 to Heroku-18. The Ruby version of my application is 2.0.0 however the Heroku-18 stack is based on Ubuntu 18.04 requires a higher version of Ruby. As my application uses an older version, upgrading is complex and time consuming. With that said,

1. Can I still be able to run with the same older versions of Ruby, Ruby Gems and Rails on Heroku-18 stack at my own RISK? As I am still be able to run my application locally on Ubuntu 18.04
2. Does heroku supports SSH to servers for installation of older versions and related plugins like AWS?