Can points be awarded retroactively?
The short answer is no, points cannot be awarded retroactively.
XP observes the actions as they occur, anything that has happened prior to XP being used is not taken into account.
This also applies when modifying the rules. Changes made to the rules won't be reflected to the users who already performed an action matching such rules.
Note, however, that it is possible to manually add and remove points to learners. You may decide to do so when your rules were too generous, or not enough.