As pointed out by aeismail, every journal have different policy. Usually, the journals/conferences accepting extended papers mention such things in there Call for Paper as well.
For example, this is a quote from CFP of Special Issue on Emerging Systems and Applications for Wireless Health Computing
All authors who presented papers at Wireless Health 2013 are invited
to submit extended versions of their papers to IEEE TETC for inclusion
in this special issue. Given that the Wireless Health 2013 proceedings
will be published, it is required that submissions to the special
issue have at least 30% new content.
Though, it is not very clear what exactly 30% new content means here, but it does indicate that your extended paper should have at least some new significant observation from may be at least the same results of your original paper.