Adapter plates should be fine. I have mine mounted on a 100x100 -> 300x200 adapter on a monitor arm so going from larger mount to smaller will be no issue.
nice. i was under the impression that those plates were to adapt a larger tv to a smaller mount, not the other way around.
though i suppose in hindsight, it should work in either direction. i just have a curved tv so its got big standoffs on it. for some reason i was thinking those standoffs would have to be used with the new tv too.