If we find new particles and also come up with a theory that extends the SM to describe them, that new theory _becomes_ the standard model. It's a moving target that basically represents the best picture we have to account for everything we've seen so far. There are already a few things already known to be wrong with the standard model, for instance we don't have a clear mechanism for the neutrino masses and you could argue that it's missing a dark matter candidate (assuming DM can be described by particle physics to begin with). The exciting thing about this (potential) 750GeV resonance is that it would indicate new physics that isn't already known to be missing from our picture of the SM.
> If we find new particles and also come up with a theory that extends the SM to describe them, that new theory _becomes_ the standard model.
I don't know enough history to know if this was the case in the 1970s or 80s, but I strongly suspect it wouldn't play out this way now. The existing "standard model" has been so stable for so long (30 years or more) that it is now treated as a very specific thing. When theorists talk about possible expanded systems with additional particles, they tend to give them names: the "MSSM" is the "Minimal Supersymmetric Standard Model", for example, and if that proved to be an accurate description of our universe I think it would carry the MSSM label forever. I expect that even decades from now when we (hopefully) have a well-established broader model in place, the term "standard model" will still refer to the same low-energy sector of that theory that it does today.
