Failed apt-get upgrade from 6.1 to 6.2 due to manually changed /etc/xwiki/xwiki.cfg file

On XWiki instance which is installed as a .deb package (see: http://platform.xwiki.org/xwiki/bin/view/AdminGuide/InstallationViaAPT ) I got upgrading errors because of changed content of /etc/xwiki/xwiki.cfg file. Actual lines I have changed were:

xwiki.cfg:xwiki.authentication.validationKey=...
xwiki.cfg:xwiki.authentication.encryptionKey=...

which I did because of security reasons.
I could only solve this by purging all xwiki-enterprise-tomcat7-mysql and containing packages and by forced reinstalling them.
Because of this many packages uses *.local configuration files which are ignored by package manager, but are used by application to override default values. Probably this should be introduced also for XWiki.