I started in 1066 as France and cheated myself the title of Latin Emperor because I wanted to get as much time as the LE as possible. However, I can't change the laws on crown authority or the laws on succession. The authority laws are all locked and my succession is Agnatic-Gavelkind. Please help before my guy dies and I have to waste time getting the land back.
You should be able to change succession laws for any title. Hover over the "?" to find out why. I haven't played a titular title (that is, without de jure provinces) since the last patch or so, so I can't say, but there should be conditions to meet like no vassals having negative relations with you, having ruled for ten years with this title, and being at peace. Some of the laws have prerequisites for crown authority, but you should be able to move to seniority or elective. Primogeniture requires high authority.
Crown authority requires a de jure kingdom or empire, though you should still be king of France even if you cheated in addition to being Emperor. So click on the French coat of arms at the top of the law screen and see what you can do.