Sia (SC) is forking on October 31st, 2018. This fork is a little complicated because it arose out of an argument between users, miners, devs, etc.
Instead of me trying to explain the fork and the spat surrounding it (it is actually an extremely complicated and long winded story), I think it is better to just point you at explanations written by people who work on sia. SEE: Sia Proof-of-Work Reset and Sia Fork FAQ.
In short, details aside, you should be aware that the best way to ensure you get both versions of SC after the fork is to have your SC in a wallet where you control your private keys.
If however you have your coins on an exchange (the fork is happening today, so it may be too late by the time you see this), you will end up with the SC that your exchange supports. In general we can expect exchanges to support either both SCs or the dominant one (my guess is this will be the one that allows all miners and doesn’t block off some).
Suffice to say, while SC is a great project… this is all a bit of a mess.