I don't think laws of succession shouldn’t be possible to change like other laws as they only apply occasionally, and it’s when the ruler’s dead.
I think one should usually start with succession laws in a normal CK2 state, but changing it should be a troublesome process. If one wants a new heir, you should proclaim one and demand your vassals swear allegiance to him. In the law screen, this would create some sort of “disputed succession” thing where (at least) two different succession laws were indicated as being arguably valid. The prior heir, who would be very likely to be pissed the liege has snubbed him, has a strong chance of forming a faction to be launched upon the liege's death (or less likely, to rebel to be named heir, or still less likely, to replace the king himself) where he would be made ruler rather than the liege’s chosen heir. Note that other people might form the faction and then try to invite their chosen person in, who’d usually though not always be the prior heir.