depends on where you are going to, ideally you would go for 2.6. the vast majority of gems is compatible with 2.6 so it would mostly be fixing internal code.

the main change I remember would be the Fixnum/Bignum disappearing in favor of the Integer parent class [1][2][3] (which isn't hard to fix).

also if you are stuck on 2.3 you might have a lot of other issues too (like outdated gems).

[1]: https://www.ruby-lang.org/en/news/2016/12/25/ruby-2-4-0-rele... [2]: https://www.ruby-lang.org/en/news/2017/12/25/ruby-2-5-0-rele... [3]: https://www.ruby-lang.org/en/news/2018/12/25/ruby-2-6-0-rele...