Forum: Ruby on Rails upgrade problems and questions

I'm trying to use the rails_upgrade plugin to upgrade a 2.3.4 app to 
3.2.9
and running into all kinds of things that "don't work".  I'll start with 
a
specific problem.  Hopefully you folks can help me discover a "pattern" 
so
I don't need to post to this group for every problem.

The "first" problem: Based on the activemerchant documentation, the 
2.3.4
plugin should be replaced by a gem.  I removed the plugin and added the 
gem
to the Gemfile.  Bundler runs clean.  But when I then try rake
rails:upgrade:check again, I get

uninitialized constant
ActiveMerchant::Validateable::HashWithIndifferentAccess