Are there services out there that allow you to easily upgrade or downgrade?

I noticed that in many different types of products and services that offer a few different subscription plans, upgrading is easy. Downgrading, on the other hand, is a bit discreet. In most of them, you would have to contact support to downgrade or cancel your plan.

While this is a business decision, are there other negative effects if we allow users to easily downgrade to a plan? Or make canceling a plan less hidden?

And this may be uncommon - but what if a user decides to upgrade/downgrade/then upgrade again within a span of, say, an hour. In general, will that cause billing issues?