Hey, Andy. So far, there has been no information about why & how and it's driving me bonkers!
Yes, the new cliff shot is only present in HD broadcasts and was not released commercially. (Someone here said that it's on the 2003 release but that is incorrect.) From the reports in this thread, it was shown on both the USA Network and Sci-Fi Channel in May 2008. I also saw it on BBC3 in August 2012.
However, there is a possibility
that the change was made sometime well before 2008. The first HD broadcast of "Raiders" was on the now-defunct UPN (United Paramount Network). They started showing HD programming in c.Oct. 2003 and the channel folded in Sept. 2006 so it would've aired sometime between then. It's possible
that this version is the same because a Paramount channel would be the likely place to premiere such a thing. (If it's not
the same, then that would make *3* different HD versions of "Raiders".)
If anyone out there has the UPN broadcast, could you check out the cliff scene, please?