"Most electrical codes allow you to grandfather in old equipment"
I am not aware that ANY actually do that.
It is the AHJs that allow "grandfathering", and could if the whim took them, require the change.
Admittedly there would be a LOT of pushback on that.
If you look at it strictly from a safety perspective, they ought to either be enforcing "it" across the board, or not at all.
{regardless of which "it" you are talking about}
If an AHJ can not figure out if there is an "inst" function, [and if it is turned on], are they going to be able to figure out if there is a ZSI function, AND if it is turned on, AND if it is wired in,[And if it is actually functional] or any of the other options.
Additionally should there not also be a performance testing requirement similart to the requirement for GF at service entrances?