How to inform a user of (possible) compatibility issues?

Question

Situation:
We've recently updated one of our software features with new functionality. Most of the source code for that feature has been rewritten with compatibility in mind, meaning that if the user upgrades to the new version, he will only see the awesome new features.

The conversion works for 98% of the users. The other 2% will have done something that was possible due to "errors" in the previous design which has been fixed, and they will have to update their settings before the feature will work again.

How do you inform those users that they should check their settings after upgrading?

I see some multiple solutions and I was wondering what would the best course of action be.

1. Make a note in the change logs, stating that every user should check their settings after installing
2. Show a one-time only pop-up after launching the program after the upgrade
3. Show a one-time only pop-up after using the feature for the first time after upgrading
4. Don't inform, it's only 2% of the user base and they made use of design-errors that shouldn't be possible in the first place.

Answer 1

The best route to take in my opinion is as follows:

Do update change logs

Good developers do read those prior to upgrade and proper expectations will be set to ensure a successful upgrade. Also, depending on the org structure, upgrade team may provide visibility to necessary teams within the org who will be affected.

One time popup

It does not matter if you are affecting 2% or 90% of all customers.

Q: Are changes / bug fixes global to the application?

If changes are global - you are right, display one-time popup / wizard walking the admin user through changes made and the settings / functionality affected. Graphical interface does help outline all small or large changes regardless whether or not the client was affected. Let the client decide if they were affected or not. Don't make assumptions on your end.

If changes are not global and are profile specific - You may need to show the same (maybe even less technical version of the upgrade wizard) to each user so they can make their changes or simple acknowledge the changes.

Automated Changes

This may be a good idea in most cases if you are "replacing" configs and you know the change is 1 to 1 in terms what the user is expecting as a result from such setting.

However, in many cases configuration or settings add new options or features or functionality where you can not make any assumptions as to what options may be applicable to your customer configuration. In such cases the upgrade wizard should be used to educate the user on newly added options and settings and allow the user to provide correct configuration changes.

Answer 2

As you state in the comments above:

if the user uses some "illegal" setting, it will change to the nearest possible "legal" setting.

Since you are already detecting the issue and doing this, then you should notify only those users who are affected.

You might want to also have a release note that warns users before they install the update (if they might choose not to update it, or there is any action they can take to mitigate the issue).