Per the comments to rand al'thor's excellent answer, I just wanted to go ahead and post an answer to say
Katie Bell also changed years
(But there are conditions)
In the books, Katie Bell is not in Harry's year. She is already on the Gryffindor Quidditch team when Harry joins it and it would be hard to understand all of the fuss if Katie Bell had only just joined.
'Seeker?' he said. 'But first-years never - you must be the youngest house player in about -'
'- a century,' said Harry, shovelling pie into his mouth. He felt particularly hungry after the excitement of the afternoon. 'Wood told me.'
Harry Potter and the Philosopher's Stone - p.113 - Bloomsbury - Chapter 9, The Midnight Duel
What's more, we never see Katie Bell in any of Harry's classes (including first year flying lessons), we do not see her getting sorted and she is never mentioned as one of Hermione's dormmates. Now, this would be understandable if she were a non-entity (I know there's a lot of speculation about the real size of Harry's classes and whether there were a lot more Gryffindors in Harry's year than we ever see), but for a significant minor character (if you see what I mean), it seems almost inconceivable.
In any case, it is finally confirmed beyond all doubt in Harry Potter and the Half-Blood Prince, where Harry is in his sixth year:
They returned to the common room, which was empty apart from half a dozen seventh-years including Katie Bell, the only remaining member of the original Gryffindor Quidditch team that Harry had joined in his first year.
Harry Potter and the Half-Blood Prince - p.167 - Bloomsbury - Chapter 9, The Half-Blood Prince
And yet she is clearly seen in Harry's Potions class in the movie of Harry Potter and the Half-Blood Prince (thanks to rand al'thor for the image):
(Katie Bell is highlighted in blue).
It is quite hard to see why she would be in Harry's sixth year class, unless she was a sixth year in the film version.
She is also shown wearing school uniform during the Battle of Hogwarts in Harry Potter and the Deathly Hallows Part 2, implying she was still at school when those events took place. This could only put her in Harry's year.
However, as I say, there are conditions. The film version of Harry Potter and the Philosopher's Stone also has the fuss about Harry joining the team:
Ron: Seeker? But first-years never make the house teams. You must be the youngest Quidditch player in a -
Harry: - century. According to McGonagall.
And, again, all the old arguments about never seeing her in any of Harry's classes (even when new characters like Bem were literally written in) until the Half-Blood Prince apply.
In the first two films, Katie Bell is played by a different actress. Then, she is largely written out until the sixth and seventh (second part, I believe) films, where she is recast and given more prominence. I think we can safely assume that the change of years happened for Harry Potter and the Half-Blood Prince, in order to highlight a character who had a relatively major role to play in the plot of that film; but who had until then (in the movies, at least) been a very minor presence.