First: You are not allowed to cancel an action if you have already used the DB from the parry of that action.
Second: We don't allow cancelling due to SAC (tell me how it works out!), only ordinary cancelling. The cancelled action has to be at least 60%. If you then choose a melee attack (to be conducted in the deliberate phase), that attack gets a -40. No bonus for a "full melee" attack, and I think no bonus for deliberate phase. We have discussed whether previous action should give a penalty or not (if you planned to move 20% during snap then attack with 80% in normal, you move in snap, then cancel your attack, and you get a -20 in ADDITION to the -40 for cancelling action). Also, after cancel action you can EITHER move OR attack (and both only in the deliberate phase), so we DON'T allow "react and melee" after a cancelled action.
But given all this, YES, we allow a change of OB/DB split. For instance, if you planned an all-out attack against the orc in front of you in normal phase, and he was killed before you could act, you can cancel action and IMMIDIATELY declare what your new action is. If that is a full parry against the ogre you see charging you, that's fine. Apply all penalties, calculate what's left to parry with (probably not as much as you'd like), and the parry is valid from the time of declaration (only against the declared opponent, obviously).