#1
Is this possible? I bought a 2008 American Start and was wondering if I wanted in the future, could change from SSS to HSS?
#2
I think most Strats are now routed for HSS or HSH under the pickguard. Next time you change strings, open up the pickguard and check. You'll still have to replace the pickguard though. If it isn't already routed, you'll have to route it yourself or get someone else to do it for you and still get a new pickguard.
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#3
American Strats are routed for HSH, so you can put a humbucker in it if you get a new pickguard
#4
I'm almost positive that Fender rout all of their strats to take a humbucker at least in the bridge, possibly in the neck too if you ever want to have an HSH setup.

You will need to buy a new pickguard though as has already been said.