Many users have been complaining of difficulty posting to Instagram via the Windows Phone Instance app.
It seems the issue, where the app would claim "Your username or password has changed." was caused by both Instagram and Instance. Instagram demanded that clients logging into the service were at least version 4.0, while Instance was spoofing 3.4.4.
Once this problem was addressed the Daniel Gary, the developer, upgraded the keychain used to authenticate the app, meaning many apps reported the username or password issue.
The solution is easy – simply restart the app a number of times, or even uninstall and reinstalling the app – both seem to fix the issue.
The issue of course highlight the danger of unauthorized 3rd party apps for online services – the high likelihood that something will break, but hopefully committed Windows Phone developers will keep interruptions to a minimum.