Sometimes a reformer comes up to a spot where there once was a fence, which has since been torn down. They declare that all our problems started when the fence was removed, that they can't see any reason why we removed it, and that what we need to do is to RETVRN to the fence.
By the same logic as Chesterton, we can say: If you don't know why the fence was torn down, then you certainly can't just put it back up. The fence was torn down for a reason. Go learn what problems the fence caused; understand why people thought we'd be better off without that particular fence. Then, maybe we can rebuild the fence—or a hedgerow, or a chalk line, or a stone wall, or just a sign that says “Please Do Not Walk on the Grass,” or whatever it is that we really need.
Why? Are they the same problems? The problems of not having a fence vs. having a fence? Are the false positives and false negatives symmetrical? That would be a massive claim! (Normally, by risk mitigation, they are asymmetrical; the unfortunate reason why we have anxiety in life, and that stupid smoke alarm sound).
Building a prior fence: Maybe the fence was taken down, cause there was no risk of flooding, and it was blocking the animals? Do we have a reason to build now or not?
Chesterton's Fence: We should not take a fence down, because maybe there will be flooding? Or it will not offer protection from an invading army, and everyone will die? We don't know, lets not risk it.
There NOT being a fence, is not evidence, that there shouldn't be a fence (even knowing there was or was not a fence there)...go out and see, should there be a fence or not?
One action requires refraining from destroying a possible benefit; the other leads to action of building a fence. You always normally need a good reason to build a wall. There having already been a wall there helps, but is not dispositive by logic, because as you say, the wall is down, and we have to speculate why it was there or why we should build it now.
The upsides and downsides are very different in your scenarios.
The inverse of Chesterton's Fence is this:
Sometimes a reformer comes up to a spot where there once was a fence, which has since been torn down. They declare that all our problems started when the fence was removed, that they can't see any reason why we removed it, and that what we need to do is to RETVRN to the fence.
By the same logic as Chesterton, we can say: If you don't know why the fence was torn down, then you certainly can't just put it back up. The fence was torn down for a reason. Go learn what problems the fence caused; understand why people thought we'd be better off without that particular fence. Then, maybe we can rebuild the fence—or a hedgerow, or a chalk line, or a stone wall, or just a sign that says “Please Do Not Walk on the Grass,” or whatever it is that we really need.