Users that currently have APK with version code 16, which targets SDK 22 or lower, will be eligible to upgrade to this APK. However, once users upgrade to this APK, they will be unable to upgrade to APKs that target SDK 22 or lower.
Tip:
If this is an intended change, then ensure that all future updates for the users that upgrade to this APK target SDK 23 or higher.
It is not related to the minSdkVersion value. You don't need to increase it unless your app is only supported by Android 6+.
The warning tells you that once you switch the targetSdkVersion to 23+ you cannot switch it back to anything below 23. This is meaningless as the minimum value is 26 anyway.