Well, I'm not an intellectual property lawyer, but ...
The references to copyright in the various editions simply denote ownership of the rights in the work. Copyright is an asset and can be transferred just like any other asset. Since Tolkien, as the writer of LotR would automatically have been the original owner of copyright in it, he must have transferred it to his publishers. Whether it has now been transeferred by them to his estate, or whether the estate simply owns the copyright in the 50th Anniversary edition, I do not know.
In any event, I would doubt that copyright could be renewed simply by publishing an amended version of a work as this would effectively allow the right to be preserved indefinately (and therefore enable copyright owners to get round the applicable law). Perhaps copyright in the new version exists independently of copyright in the original, or perhaps it simply lasts as long as copyright in the original. The latter would produce a less complex situation but, then again, lawyers and lawmakers have never been known for eschewing complexity.
I am sure that someone who specialises in this field would be able to explain better than I. Athough I could always do some research into it (for a suitable fee, of course
).