Yes, it's a pretty big deal, you would want to be a machinist and have access to a milling machine.
And before you buy some of them, the detachable "duck-bill" mags for the standard SKS are useless and a waste of money.
Just buy a "D" model if you want one, they are not much more expensive then the standard model.
Personally, I think the standard model is superior anyway.....bolt hold open on last shot, you can load with chargers which is just as fast or faster then rocking in/out an AK mag, chargers are about $1 a piece or free with czech ammo while a 5/30 AK mag is $60+, standard sks is more reliable, standard SKS generally comes with bayonet while the "D" model doesn't, parts are plentiful for the standard model, not so much for the "D"...........etc. etc.