Upgrade vs Increase
When you upgrade the difficulty you change a to a . If you run out of to upgrade (ie. the pool is only ), and still have extra upgrades remaining, first add a new , and then if there is still an upgrade remaining, convert that to . Positive dice may also be upgraded, following the same rules - to , and add if needed.
Example: If the pool is upgraded once for both the positive dice and the difficulty, it becomes . If the check (positive dice) and the difficulty (negative dice) are upgraded four times that initial pool would instead become .
To downgrade either the check (positive dice) or the difficulty (negative dice), it only applies to or . After all dice have been upgraded (if applicable), downgrade by replacing with or with . If more downgrades are called for than there are or available, ignore the remaining downgrades.
Example: Using the final pool from the upgrading example, downgrading both the positive dice and the difficulty of the check four times each would result in - the remaining downgrades are ignored. So downgrading does not just offset upgrading (ie upgrading 4x and then downgrading 4x does not cancel each other out to become the initial unmodified pool).
When you increase/decrease the difficulty you add or remove . This is done before any other modifications to the dice pool. In an opposed check it is assumed this would be the equivalent of increasing/decreasing the highest of the opposing character's characteristic/skill level. Upgrades would then be added to the pool (in opposed checks the lower of characteristic/skill upgrades the difficulty), followed by any downgrades.