I think some companies will just cede the California market. It's already happening with smaller manufacturers, and the more complex the rules get, the fewer choices Californians will have (unless they're LE, and the rules for mere citizens don't apply).
I don't know how easy or difficult it would be to retrofit a design that doesn't have a mag safety as part of the original design. Presumably it would be easier on some guns that others, but I strongly suspect it would be more difficult on small gun versus larger ones.
Kahr isn't a huge manufacturer, and I don't know if their California sales justify redesigning their guns - and perhaps making them less reliable in the process. I'd get the PM9 sooner rather than later, if I were you.
I certainly hope Glock and some others don't muck up ALL their guns with useless extra "safeties" just to appease the masters in California. Springfield seems happy to tack on afterthought safeties, though, so maybe XDs will still be available after the deadline.