If you have no legislature, then you should be able to change your legitimacy type (and thus your available succession laws) by decree. At the moment, you can't change legitimacy type if you do have a legislature.

The most effective way to change your power base (prefrred holding type) at the moment is to get a specific holding type into your demesne, then give away all holdings of your current preferred holding type. You'll then be asked to either change power base, or steal a holding from one of your vassals.

ahh ok, ill give that a try, thanks mate