It was very common for owners to keep on wearing tunics through promotions and this is not at all unusual to see. Simple logic comes into play here too; why go to the expense of buying a whole new tunic when you are just changing the rank?
Somewhere in one of my many books I read about one of the army field marshals who took it to quite the extreme and wore the exact same tunic, every day, to the point where it was not too pleaseant to be around him (I think he was not a regular bather either). Evidently it was pretty ratty and had been continually patched up and repaired over the years by his adjutant. He even wore this when called in to see Hitler, much again to the dismay of his adjutant. Don't remember which GFM it was, perhaps somebody else can contribute the name in the meantime.
