unique civ bonus idea

What do you think of a civ getting a bonus where you get all built palisade walls turned into stone walls? The stone amount will be reduced from your current total and construction time takes somewhere between a stone wall and gate (10-70 seconds). This ability would be able to be used indefinitely as long as you have enough stone and palisade walls/gates built. On one hand you would lose a layer of palisade walls if you build stone walls behind them but it is much faster than having your villagers build all those walls. It would save a lot of villager construction time kind of like the hun bonus with houses. You would have one villager activate the ability and if the villager dies then the walls don't get converted.