Man, this might be my favorite Mystic card. I expect it to get Taboo'd at some point, maybe with that replacement ruling reverted back to the original, because an essentially free 14 XP (because you always take two) is just too easy to take in this game. Even Agnes should consider it despite the anti synergy with her ability- her Mystic/Survivor card pool gives her decent access to horror soak, heal, and prevention (you can always try and infinitely recur St. Hubert's Key with Scavenging for example) which makes the 2 horror she starts with not too big of a problem. But let's check in now and see what spells we can upgrade to as of The Circle Undone wrapping up:
Blinding Light Blinding Light 2 for 0 XP
Clarity of Mind Clarity of Mind 3 for 1 XP
Deny Existence Deny Existence 5 for 3 XP
Mind Wipe 1 Mind Wipe 3 for 1 initial XP
Mists of R'lyeh Mists of R'lyeh 4 for 2 XP
Rite of Seeking Rite of Seeking 2 Rite of Seeking 4 for 0 XP
Scrying Scrying 3 for 1 XP
Shrivelling Shrivelling 3 Shrivelling 5 for 1 XP
Sixth Sense Sixth Sense 4 for 2 XP
Ward of Protection Ward of Protection 2 Ward of Protection 5 for 1 XP
Wither Wither 4 for 2 XP
Also in Return to Carcosa we're getting a lower level Suggestion, so look out for that Sefina Rousseau and Akachi Onyele.
So which of these upgrades do you shoot for? Remember that you only get the benefit on the first spell you buy, so a chain of three cards like Shrivelling to Shrivelling 5 is going to take 4 scenarios to reach if you want the full discount. In other words you'll get the benefit 7 times in a standard campaign (though side stories will help you out quite a bit). There's a few up there that stand out to me:
Obviously if you're going to be a fighty Mystic it's a good plan to move through the Shrivelling chain. However you might find you have XP to spare in which case it's probably better to spend the XP for the other upgrade sooner, making you more powerful more quickly (and increasing the liklihood of picking up enemy Victory points later). Instead, I recommend using AR's savings in other areas.
If your name is Diana Stanley or Agnes Baker then I strongly recommend Deny Existence 5. If you plan on running Diana with Dayana then I even more strongly-er recommend it.
Mists of R'lyeh upgrades into a big +3 bonus and an added charge. Particularly good for Jim who is just a bit lower in than anyone else, and also valuable for that one campaign we all know where evasion is super useful.
Ward of Protection 2 is usually considered superior to its overcosted counterpart upgrade Ward of Protection 5, so probably just go up the first step in this chain and then stop. This is especially true in higher count multiplayer, and especially extra super true for Diana in multiplayer. In solo with a less fight capable mystic you could eye 5 instead, or else skip the chain entirely.
Jim should probably always take Sixth Sense 4 since it becomes a very consistent multi clue gathering tool for him, assuming you're building him in the traditional manner of drawing more spooky tokens.