In practice just giving the ranger full AC progression and taking nothing away is usually a fine move since many rangers could use the extra bump in power. As for it being a "fair" trade to swap the combat style for a better AC, a good AC will definitely outdo the feats granted by the combat style. So no, it's probably not "fair" in the sense of an equal trade, but outright giving the ranger a druid's AC is one of the typical houserules to balance the classes (and dropping the druid's AC to ranger's progression.)