SKS's are not the most reliable rifles in the world...
Also look for how it affects the compliance - 922R or whatever the number is.
Wise words. Merely changing the magazine on a C&R firearm like the SKS violates Title 18 USC § 922(r), meaning you'd now possess an illegally modified gun. 922(r), like almost ALL gun laws, is stupid and makes no sense. However, like all laws, it's the law, and you can get busted, bigtime, for breaking it. It is also complex, and you need to make sure you're following its many idiosyncracies should you decide to modify your SKS.
Take a look here for a ton of information on the subject: http://www.sksboards.com/smf/index.php?board=12.0
If by merely you mean only, that's true. However, if you go through all the parts counting nonsense, you can legally swap the mag out for a 30rd.