Under what circumstances can one republish a conference paper/presentation? For conferences that "peer review" and "publish" full papers, when, if ever, can you resubmit essentially unchanged versions for publication in a peer reviewed journal?

In my field, when conference papers appear as book chapters, people often "republish" them as journal articles.

I have just had a talk accepted at a conference which is now (decided post submission) planning on publishing the proceedings as a special issue of a journal. It appears the journal is complete crap with an extremely light peer review process in general, no impact factor, and essentially not indexed. I think this "article" will be worthless, but I am concerned that I will not be able to republish the results in a respectable journal.
This varies by field. In (most) fields that have journal-based publications and no tradition of selective refereed conferences, it might be quite hard to publish a paper in a journal after it has appeared in some proceedings, without substantial changes.

In computer science, in which conference publication is the norm, it is expected that journal papers have first appeared in conference proceedings. This is especially true if the full paper does not fit in the page-limit of the conference proceedings, and proofs or other material had to be left out of the first publication. Typically you have to make some small changes between the conference and the journal version (i.e. include full proofs, include a fuller discussion of related and subsequent work, etc), but in CS, the delta between conference and journal version can be relatively minor.