In general, the krb5 configuration should respect values already in /etc/krb5.conf if there is an existing krb5.conf on the system, and the values from that file will override preseeding. That's according to debian policy and I can look up the reference if you'd like.
However, if there is no krb5.conf, values from the debconf database should be used and preseeding should work fine. If you purge the package, then the file should be removed, and so should your preseeding. purge package debconf-set-selections install package should behave the same as a fresh install. If the above is not the behavior you're seeing, please clearly explain which case fails to do as I've described and I'll be happy to look into it. If you think the above behavior is not what is required by debian policy, also let me know and we can discuss. --Sam
