When you upgrade to a new Tier in a Domain or gain a new Domain, you:
Optionally you may swap one of the cards you previously selected for a new card.
After upgrading your Attributes and adjusting any affected Aspects, you will need to recalculate your Action Point Bonus.
Your Action Point Bonus is equal to the sum of all your Tier levels divided by the number of Domains, rounded down.
$$ \text{Action Point Bonus} = \left\lfloor \dfrac{\text{Total Tiers}}{\text{Number of Domains}} \right\rfloor $$
$$ \text{Action Load (when reset)} = \text{0} - \text{Action Point Bonus} $$
Example 1: a character is Tier 2 in Skirmish and Tier 3 in WeaponMaster. $$ \text{Action Point Bonus} = \left\lfloor \dfrac{\text{2+3}}{\text{2}} \right\rfloor = \left\lfloor 2.5 \right\rfloor = 2 $$
Example 2: a character is Tier 3 in Fury, Tier 2 in Skirmish, and Tier 2 in Incantor. $$ \text{Action Point Bonus} = \left\lfloor \dfrac{\text{3+2+2}}{\text{3}} \right\rfloor = \left\lfloor 2.333... \right\rfloor = 2 $$