You need to extend it with atleast 30% new content. If the paper is interested it is easy to extend it with more content. Any problems ? No if more than 30%
Republishing something is always a bad idea, and when done it exposes an obvious intention of the author to "pad" one's list of publications by adding titles. The same thing applies to splitting the findings of a significant study into multiple, largely irrelevant publications.
You should instead write a new paper, and avoid repeating what you have already written in the conference proceedings. Just add a reference to the conference proceedings instead. The new paper must be scientifically relevant on its own, of course.
Usually, several papers of a conference are chosen by the conference organizers for publication in a journal. They write a letter of invitation, and suggest additions or editing work on the paper. if you are sending a conference paper to a journal without the aforementioned process, than you should edit it so that it will fit the journal's instructions for authors.
Actually I have had different experiences.... I would encourage folks to publish Proceedings from an important Conference &/or findings for wider exposure ( not to pad my publication at all..... not necessary). We are building a FREE Digital Archive at the University of Maryland around material related to our field.... it is not necessarily the same as an official publication... but the intent is to create free access to information in our field..... We are now working with two other Archives in the US... same intention to help professionals have access to information they may not have been able to hear at a Conference etc.... Depending on the Field, there are various ways to publish your Conference Proceedings.... Good Luck!
I believe for wider access and reach, the conference paper could be sent to a journal for publication. However, it must be edited and updated and/or expanded to suit the aims of the journal. It is professional to add that the paper's content has been presented at a previous conference.
the general rule is a published paper can not be republished. but it is a working paper which you have sent to the conference, of course you can develop/ update/expand it and publish in a journal. but you have to disclose that the early version of the paper has been published in this conference.
It depend to the given journal in which you wanna submit the paper; some of journals admit such a submission. However, you should mention the former venue in first page of manuscript.