I haven't been able to actually find patch notes that say this, only a few posters talking about the change in beta. From what I gather spells switching from per rest to per encounter at later levels is gone. In place is a system that allows for mastery of one particular spell that can but used per encounter and other spells remain per rest.
I apologize in advance for not having the time to write out a very detailed post on the pros and cons of per rest vs per encounter. It has been debated at length on this forum though and of the argument put forth I would like to simple add this.
I agree with nearly every point that supports a per encounter system, and find the arguments for the vancian spell system to be antiquated.
Perhaps most importantly for me, the per encounter system is simply more FUN. Also the per rest system FEELS like I need meta knowledge of the game (like what mobs are around the corner) to use spells correctly.
So this post is simply a plea from someone who has sunk over 200 hours into this game, please don't move closer to the per rest spell system.
If anything I have been looking forward to having more spells become per encounter as my casters in all my different parties move into the final phase of the game.