What do you call it when the arrival platform of a train changes?

Solution 1:

In British railway announcements in English, this is always called a platform alteration, as your video — apparently shot at Lincoln — demonstrates.

That shows the standard recording, with the different parts of the announcement spliced toegther electronically. The phrase "This is a platform alteration" is always used in these situations.

Chris H has commented that in Welsh the phrase used translates to newydd platfform (literally “new platform”) and it’s possible that the idiomatic phrases in Gaelic differ too.

Solution 2:

In North America, it's called a platform change or just a change of platform.

At least from Google Books Ngram Viewer, platform change is the more common in printed form. (And platform alteration doesn't exist at all.)

I will also say that I have never heard platform alteration before—either where I am in Canada or at any time that I've travelled to the States. (I have also been to the UK, but have never actually heard any announcement of a change—or alteration—at all, so I can't personally speak to the wording used.)