We prepared for something like this by adding an extra feature to the SC1. We can do a soft-fork that slightly changes the PoW algorithm which would invalidate the bitmain ASICs, but allow the SC1 units to continue working.
The Sia community over on Reddit has basically split down the middle on whether or not to accept the A3, but because there's no major consensus NOT to... you'll be fine. The Sia dev team has come out and said that they're welcoming Bitmain to the ecosystem and will be "watching" them to make sure they're not a bad actor in the community. So until (or if ever) something bad happens, they won't be doing anything like that.
While it was a shock to them all that Bitmain has done this, the loose consensus is that it's a good thing in the longer-term.
Honestly, I'd never even heard of the coin until the A3 announcement...so if nothing else, Bitmain has brought a ton of attention and awareness.