Changing the sights probably isn't an issue. The key is whether you "assembled an unimportable firearm from imported parts." An otherwise importable SKS with Techsights would still be importable, UNLESS it was an otherwise unimportable SKS (w/bayonet, for example) that was imported on a C&R exemption, in which case changing the sights could conceivably remove its C&R exemption and make you run afoul of 922(r), unless the gun is more than 50 years old, in which case it's still a C&R even with Techsights due to the 50-year rule. (Confused yet?)
The problem with 30-round SKS mags is, if BATFE considers all 30-round SKS's to be unimportable, then taking a 10-round fixed-mag SKS and putting a 30-rounder on it would be a felony violation of 922(r) even for a non-C&R rifle, UNLESS you play the parts-count game and replace enough other parts that the rifle is no longer classified as an import.