Advice for replacing old string inverter

[RKCRLR]

I have a newer 6kW Enphase/Panasonic system and an older (2009) 4kW string system. The system is installed on an uninhabited metal building. I'm planning on getting 2 Powerwalls. My understanding is the Powerwalls will work with both inverters but I'm concerned about the old string inverter. It went out a few years after it was installed and was replaced under warranty with a new unit of the same model (Yes! ES 4000P). It is now out of warranty. I haven't yet decided whether to just replace it when I get the Powerwalls installed or wait until it goes out. Either way, I'd like to be ready so I know what to do.
The old inverter is not a smart inverter, there no optimizers (and I don't have a shading problem), and the system is not rapid shutdown compliant. My county is under NEC 2014 for new construction.
1) Will a smart inverter work better with the Powerwalls than a older type inverter?
2) Do I have to comply with the NEC 2014 rapid shutdown requirements if I replace the inverter or am I "grandfathered" in?
3) I was looking at the Sunny Boy 3.8-US-41 inverter, it seems like it would be a good fit. Are there other/better options I should be considering for my situation?
Thanks

 

[Ampster]

What model are your Enphase inverters?
Here is a link to a thread that discusses what happened when micro inverters were installed that could not modulate. If your Enphase are IQ7 then they are able to modulate. The issue in this thread actually reveals itself on page three when the poster realized he was supplied an inverter that only turned on or off when sent the frequency signal from the Powerwall. Despite the headline of the thread the problem was not the Powerwall but the behavior of the other inverters.

That is what you might experience with your old inverter.
Misleading Capabilities of Backup Mode?

 

[RKCRLR]

My Enphase microinverters are IQ7s.
I skimmed through the thread. It will be interesting to see how my system reacts during a power outage.
My old 4kW string inverter will go offline once the Powerwalls (Gateway?) increase the frequency to 60.5Hz. But the 6kW IQ7 inverter system can be modulated so it appears that they will continue feeding the house/Powerwalls. Then, at some point when the Powerwalls get low enough, the frequency will go back below 60.5Hz and the string inverter will restart. Maybe not as efficient as total modulation but it seems like it will work. One of my concerns is how hard this will be on the old inverter where it is being shut down and restarted multiple times.