change rails environment in the mid of testing

You could do

Rails.stub(env: ActiveSupport::StringInquirer.new("production"))

Then Rails.env, Rails.development? etc will work as expected.

With RSpec 3 or later you may want to use the new "zero monkeypatching" syntax (as mentioned by @AnkitG in another answer) to avoid deprecation warnings:

allow(Rails).to receive(:env).and_return(ActiveSupport::StringInquirer.new("production"))

I usually define a stub_env method in a spec helper so I don't have to put all that stuff inline in my tests.

An option to consider (as suggested in a comment here) is to instead rely on some more targeted configuration that you can set in your environment files and change in tests.