Recent versions of Mongoid have introduced many features–semver, feature flags, load_defaults–which have made it much easier for Mongoid users to upgrade.

However, there is not a comprehensive/unified guide in the docs of how these should actually be used. For reference, here is Rails' guide: https://guides.rubyonrails.org/upgrading_ruby_on_rails.html

I will take a first pass at writing something.