It will be raised HP costs + 'memorizing' spells at certain levels.
Healing spells will definitely have their cost raised at least by 2 points, if not more. Some tactical spells (warp, rescue, sleep, etc.) will get their costs raised as well. Most offensive spells will be left intact except status-related ones (carrion or nosferatu, for example).
The basic idea of 'memory mechanic' is that when (for example) Druid first hits lvl2, he memorizes one of two spells: Flux or Fire. Then at lvl4, he can memorize either Carrion or Heal
or the spell he didn't choose at lvl2. And so on; at lvl20 he would have 10 memorized spells at most.
Of course, each PC would have their Call Magic listed in their sheet, so they wouldn't have to worry about forgetting which spells they took
