Well, genetically they don't change. But the effect those genes have on the coat colors over time can cause one to think they have a new horse every season.
I would suspect both roan and grey are present loosie. They not only effect the over all look season to season independently but put them together and they effect the timing of how grey progresses.
I have several palominos that are white yaks come the fall and then shed off to varying shades of gold. The sooty ones being more "chocolate" in appearance, the nd1's cinnamon colored where a typical dun would show primitives and a couple that you would swear were roans the ticking is so extensive when combined with nd1 and the plain Jane gold though they have a metallic sheen that makes them highly reflective. Gives meaning to the phrase "A horse of a different color".