There is actually no way to extend copyright. You get life plus 50 years neither more nor less, and that's that (unless you are Walt Disney, in which case you can ask Congress for a 20 year extension and they'll give it to you).

So your publisher won't be able to resurrect your copyright. However they could make a new edition with for example a new preface or a new page layout and there would be a fresh copyright in those changes, but not in your original words.

This U-shaped graph explains the problem: http://ip.jotwell.com/how-copyright-prevents-us-from-getting-the-books-we-want/. After the short initial short period when the peak profits are earned from the work, the rightsholders don't bother to keep their works commercially available, so they are essentially lost. This graphic covers books, but for other types of works the same has been observed—there are instances where original master tapes of music that studios thought was no longer valuable were trashed, and in consequence the best existing copies of some music are illegal copies from P2P file sharing services.

Covering the point you raise, in civil law countries (but not New Zealand) there is an independent "moral right" that is inalienable and permanent which can allow you to prevent your work being misused, even after you're no longer entitled to receive economic returns for its use.