Swapping 30 rounders in an SKS requires you to have the bolt back since the feed lips of the mag ride in grooves in the lower section of the bolt. Other than that you just rock the mags in like an AK.
Its a pain in the ass.
Hold the bolt back with one hand, hold the mag catch back with the other and use your 3rd hand to rock the empty mag forward to release it from the receiver.
You’re half way there. Its as easy herding cats