Sure, I ordered a subscription to audible through the App Store. They have 2 plans and I chose the cheaper one. Then I found out the cheaper one didn’t include what I wanted so I called Audible to fix the problem and they directed me to Apple since that’s how I subscribed. I then called Apple and they had no ability to help me other than the options listed in the Apo Store. So I ended up paying twice to get the upgraded plan. Apple said they would refund me a discount for the difference but they never did. I cancelled after a month and I have yet to order any more subscriptions from Apple.

That is a good example.

Were you not able to cancel the plan you didn't want and sign up with the plan you did?

If that wasn't an option (ie, you wanted the better plan right away), quickest might be to request a refund of the bad plan and then sign up with the right plan.

If you request a refund the same day of purchase I've not had a problem with that (but it hasn't happened often). I had a somewhat similar situation, and just noted that it wasn't clear that X was not included, had no problem getting a refund. I then signed up as desired.

That said, devs should have an upgrade option that charges difference ?

No, I could not cancel the order even the same day and when I called Apple they had the same options that I did, no better access. If I had ordered through audible they could have helped me but since I ordered through Apple nobody could help me. No upgrade option.

So lesson learned, never order a sub through Apple if you can avoid it.