This is what I found on wiki:

• Any changes made to a leveled actor's leveled (or 'temporary') base will be lost when the actor re-levels.
  
• Any changes made to a leveled actor's non-leveled (or editor) base will not show up on the actor until the actor re-levels.

Does it mean that if I make LeveledActorBase changes (to lets say - Follower) via script, they will be lost after the actor re-levels?

If so then how could I keep these changes? Or how could I detect "Level-up Event"?