I have a P320 from before the "Voluntary upgrade".
After a while I called Sig.
I asked the Rep about it.
He stated, that they did many tests and only and if only dropped pistol at a specific angle on the back left side of slide it has fired, so Sig decided to do testing and made some changes until they were sure it was safe.
I asked how many sent their pistols back for upgrade and he told me less than1/3 and the offer does not expire no matter the owner.
I decide to send in my P320 as I wanted to do some personal upgrades to my desires and I wanted all new parts to fit without issue.
I sent my P320 in and got it back in less than 10 calendar days.
Sig and other have videos to show you exactly what they modify.