Let's try this:
1: An historic building is grandfathered in to a zoning change
2: The buiding changes owners
3: The building closes for needed renovations
4: The renovations take some time
5: During that time, the city removes the historic building's historic use
6: There is no other way to use the building
7: The city offers a longer, more tedious, and possibly less successful process to amend the zoning
For a city like New Orleans, where historic character is theoretically held in high esteem, this is ridiculous. It proves the lie behind what passes for historic preservation in this town, and the joke that is zoning here. This city is in such terrible condition in many ways because there are government regulations that force it to be this way.
And that's not just some sort of Tea Party "goverment = bad" rhetoric. Regulations are necessary, but you have to make sure they work for your city and don't kneecap your neighborhoods. You have to have a process where historic buildings can be purchased and updated and renovated so those properties can maintain value. If you require specific, specialized renovations, that work is going to take time. If that work takes time, you have to recognize that when it comes to zoning issues.
Most importantly, some aspect of reality must be factored into your decision-making process. Who the F wants to buy a 14 unit aparment building off the current owners and renovate it again for one or two family living?
Here's a few scenarios:
A: This will artificially drive down the value of the property for the current owners, who have already sunk money into the project. Some well connected individual can come along and buy the property so the current owners take a loss, then use their connections to get the zoning fixed and make bigger profits.
B: This will artificially drive down the value of the property for the current owners, who have already sunk money into the project. Some very wealthy individual comes along and buys the property, tears down the historic building, and constructs something more realistic to reflect the zoning.
C: This will artificially drive down the value of the property for the current owners, so they let the property deteriorate. It won't appraise for as much, but what is their incentive to keep up a property they cannot use? As a St. Charles Avenue home, this isn't likely to happen, but anywhere else this could contribute to blight.
You'll notice that "make a reality-based decision about the property; charge the owners a small fine for being late on the renovations; allow the property to go back into commerce and increase the city's property tax base, historical character, and housing stock" are not a part of the viable scenarios here. That would be to "win-win-win" for a city like New Orleans.