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