Theoretically, before 1905, Nicholas as Tsar and Autocrat could have done anything he wanted to with the laws.
However, he was not a strong man and preferred to avoid confrontation. If his father Alexander III had chosen to change the laws of succession, I don't imagine he would have had too much trouble although he would have had opposition, he was better at controlling it than Nicholas was. And why would he want to? He had after all, three sons of his own. If one didn't inherit, then presumably the next one would.
Remember the "mobilization of two or three Austrian Army Corps"? and AIII's bending a silver fork into a knot and the saying that "this is what I will do to your two or three Army Corps."
But AIII was imposing not only in stature but in will. Nicholas was neither.
Nicholas faced opposition from his uncles even on the date of his wedding and the choice of its location. He bowed to them instead of the other way around.
Changing the laws of succession would have been a tougher confrontation with Valdimir and later Paul who had sons at the front of the line.
I used to think that Nicholas should have just done what he wanted and changed what he wanted and gotten married when and where he wanted, and even changed the Imperial tea, as Alix said that "other people have more intersting teas", but he just didn't have the intestinal fortitude.
As subjects of the Tsar, the other Grand Dukes should have bowed to the Tsar's will, but because Nicholas was so weak and indecisive, they took advantage and did pretty much what they wanted to do.
I hardly think that the people of Russia would have cared who inherited. After all, to them the Tsar was next to God and could do a he liked. It would have been his own family who would have given him the roughest time over it.
After 1905, NIcholas was no longer an autocrat, even though he didn't truly believe it, and as such could not have changed anything without the premission of the Duma.