I rode a pony for two years whose coat changed so much! In the winter he looked like a bay but in the summer he looked like a normal roan. But if you shaved him, he turned into a gray pony!!! I think that this occurrence is very normal, so don't worry!
I know its not the same colour but we have a breed here called erskays, they go from black/brown at birth to light grey as adults. I dont' know if that's of any interest to any one, in they're case it a genetics and breed standard.
Sounds like my grey dapple...now I know that grey's turn white over time but Dumas always looks best in spring. Now that it's November, he's got his winter fuzzies comming in quite nicely. I notice it in the areas you mentioned first, then he starts to fuzz into a winter white-ish full coat.