The first one feels more natural. The typical flow is:
Define a Goal
Make a decision
In your first flow the user defines their goal: "Change my plan." Their next expectation will likely be "What are my options?" and once they've indicated their intent to pick a different plan, the confirmation reassures them their goal has been reached and ...
Econsultancy did some up to date market research on this topic.
While subjective, the themes coming through in the analysis is that the use of colour is intrinsically linked to the brand values and audience. For example, Crayola uses a vibrant fun colour scheme you would expect of a crayon manufacturer, alas not for an office stationary store.
It's possibility to add UI controlers, which changing the view of displaying product:
In the case of single product page - it's not a clear and universal solution.
It depends on a lot of variables (like industry, customer type, amount of information needed)
Suggestions and ideas:
highlight and raise a section with warehouse information / availability