If you read the rules of the journals, you will see that there is actually a generally accepted rule. That rule is:
Your submitted journal paper should include at least 25% new material from the conference paper
So, you should not simply republish the same paper, rather you should add something to it.
The logic goes like this: You have a well thought out idea. You write it up and present it at a conference. During the conference, you get a discussion going about your paper and workout some details that you might not have considered originally. Then you go back, add (or clarify) a little bit (25%) and then publish it as a journal article.
This is extremely common in business management.