From Exalted - Unofficial Wiki
The topic of adding reflexive charms to a pre-existing combo that aren't used to modify a contiguous action (say Excellent Strike and Hungry Tiger Technique comboed with Shadows Over Water) was first brought up here.
While I think charms used to modify the same action necessitate a re-learning of the combo, if you're just tacking on a defensive charm or an initiative adder or something that doesn't have to be activated when you use the combo, it is a bit silly I think to have to relearn the whole combo over again (thereby wasting xp/freebies spent on the prior). Thus my rule is:
If an Exalt already knows a combo and wants to add a charm that won't necessarily have to be activated and paid for every time they use the combo and can't be used to modify another action in the combo, the cost in XP to learn this "new" combo is (minimum attribute/ability rating of the charm being added + number of charms already in the combo).
This lends itself to comboes that grant more than one type of action in a round; a melee attack and a parry, a melee attack and a dodge, etc. Charms may then be added that would modify one action, but not all actions. In this case, the cost in XP to learn this "new" combo is (minimum attr./abil. rating of the charm being added + total minimum attribute ratings of the charms that could effect the same action + number of other charms in the combo). This includes charms that have the same trigger (such as Essence-Gathering Temper and Willpower-Enhancing Spirit).
Danny the Dawn caste Solar already knows a combo consisting of Excellent Strike and Hungry Tiger Technique. He wants to add to this a defensive charm, say Dipping Swallow Defense. Because DSD can't be used to modify a melee attack, the cost to learn this new combo is 4 XP; 2 for charms already in the combo plus 2 for the minimum ability requirement of DSD. Danny now knows Excellent Hungry Swallow Technique.
He finds though that his regular parry pools just aren't enough a lot of the time. He decides to add Golden Essence Block to his Excellent Hungry Swallow Technique. This costs him 5 XP; 1 for the minimum ability requirement of GEB, plus 2 for the minimum ability rating of DSD because they both can "modify" a parry attempt, plus 2 for other charms in the combo. Just cuz' he's an ass, Danny even decides to re-name his combo: Excellent Golden Hungry Swallow Technique.