Should service animals that will be excluded in March when the legal definition changes be grandfathered?

Having grandfather clauses for animals might seem like a good solution, but I think it would be hard to enforce. We don't currently have a system of registering or certifying service animals. How would you prove that a particular animal was grandfathered in? Do we set up a temporary registry? That seems like a lot of effort to go through for something that will only be useful as long as the grandfathered animals are able to work.