Paramount is actually 97% region free. There's no reason to think that it would change just for this title and to think so is being overly paranoid.
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If you examine a list of Paramount's Blu-ray releases, those that are Region Free are
predominantly catalogue titles or titles for which Paramount have worldwide distribution rights. However, even that is no guarantee, as evidenced by their recent release of "Hercules" which was Region A locked in North America but Region Free elsewhere, even though
all those releases were from Paramount.
The situation is radically different in the case of titles for which Paramount only has the distribution rights in North America, with the rights elsewhere being held by other studios. In those cases, Paramount have
tended to lock their US and Canadian releases. Examples of where this has been the case include the following titles:-
"Titanic"
"Braveheart"
"Gladiator"
"The Wolf of Wall Street"
"Boyhood"
"The Curious Case of Benjamin Button"
"AI: Artificial Intelligence"
"Minority Report"
"The Fighter"
"The Road to Perdition"
The above list is not exhaustive but does support my contention that Paramount's US and Canadian releases of "Interstellar" are
likely to be Region A locked since Paramount only have the North American rights to this title, the rights in other territories being held by Warner.
I fail to see, therefore, how a logically reasoned argument can be thought of as
"being overly paranoid". If anything, given the shared distribution of this title with Warner, it would be Paramount
not locking this title that would represent a change in policy.
I also stand by what I wrote earlier, that relying on a pre-order listing on an import site, to determine a disc's region coding, is fraught with danger, given the recent example of "Hercules", especially with a title not due to be released for another two months. The only definite way of
confirming the region coding of this release is an announcement from Paramount or actually testing the disc, once in hand.