also the choice to make bren's first on screen magic transmutation immediately pinged an interesting one, given that we knew he had an evocation specialisation first, and deliberately switched to transmutation because prior to dunamancy transmutation was the school most concerned with time, and he wanted to go back and fix his mistakes
but seeing the rest of the episode makes it pretty obvious what symbolism they're leaning on there, and it's a very fun one!
caleb, in the campaign as well, has always had this kind of push and pull between evocation and transmutation. at their base level they represent damage and healing
evocation mechanically is about pure destruction, big but precise. your class features let you increase your damage, let you choose exactly who you want an aoe to hit, and eventually let you start drawing the energy from your own body, injuring yourself for the benefit of supercharging your spells. transmutation is about change, yes, but change for the better, it's the most support based wizard subclass, giving you a lot of buffs to cast on people and at higher levels even letting you heal (something wizards otherwise cannot do)
wizards specialise at level 2, but they can still cast spells before then. and i think what we're seeing of bren pre-soltryce is him as a level 1 wizard trying stuff out. and we already know how he feels about the two schools! they gave us two examples of spells bren cast - the magic to fix the fence (a transmutation spell), and the magic that destroyed part of the hearth (an evocation spell). he was trying to help in both instances, but the transmutation is a spell he felt good about, the evocation was a mistake he's ashamed of
but a transmutation based volstrucker isn't nearly as useful to trent as an evocation based one. so everything he said to bren in his home was to draw him into an evocation specialisation. he points out the power he could access, and the precision that training in the school of evocation could offer him. no more mistakes. he makes bren cast a fire spell as a first test, making that connection of fire is how you are useful to us, this is how you prove yourself. and of course sacrificing your own health and comfort to do insane amounts of damage is what trent's already experimenting on with implanting residuum. the transmutation, the desire to fix and to heal, trent's only interested in that as far as he can twist it
contrasting the first two spells bren cast also mirrors what he's going to do. he fucked up with the fire, he's decided to be useful with transmutation, he's using it to mend a fence. and the goal he's working towards by switching to transmutation is to fix everything! to undo the effects of his mistake
now not to say he's only used transmutation for good. his entire plan to go back in time was questionable from the start, and as people have mentioned already, we saw bren combine both when killing his parents (obviously evocation for the fire, but transmutation when sealing the doors and windows). but the intent was still pretty obviously to fix, with the time travel, and so was the murder. bren didn't do that because he hated his parents, but because he'd been taught to do anything in the service of fixing his nation, and he believed they were harming it. conversely, a lot of his invented spells are evocation, and while destructive, they're not inherently evil
but that push and pull, to destroy and to mend, what happens when you try to do one and end up doing the other, what happens when it's both, when it's neither, we see that constantly throughout caleb's character, and it definitely defines him. i don't feel like i can truly end this meta by saying he chose one over the other
choosing to switch to transmutation though, back when he first escaped vergessen, was making a commitment to something. and i think by the end of the campaign he's found how he can fix things in ways that truly matter, and how to focus on fixing things that aren't just his mistakes
(and i very much look forward to seeing more of his spellcasting visually)