I would ask the question the other way: How does replacing the roller wheels invalidate the warranty? Where's the reasonableness in that?
It's like putting mag wheels on a car and that doesn't crap out the warranty.
Not sure whether it voids the warranty, but Brompton has said that it's wheels are designed to break before the supports for the wheels. If you replace the wheels and those supports break, it seems reasonable that it be classified as "misuse".