Exactly. Normally I wouldn't bother taking Lore of Fire with swaps, but my last game had a lot of trolls so I went with it. You have to figure out what the biggest threat the other side is presenting you and/or what your own army is currently lacking.
The following are guidelines, not rules.
The first thing I’m going to say is not to get so excited about the opportunity to swap spells that you cast spells just to swap them. You can get away with this once or twice in the first two rounds but you still want every spell to aid your cause, even when you are planning to ditch the spell. There is a tendency to get distracted by the desire to build “the perfect list of spells.” At best you’ll only have your perfect list of spells for five turns. Your point is to try to win the game. If you find yourself saying something like, “I’m going to keep swapping for Death spells until I have all three heroes snipes,” then you really should be taking Death, not High Magic.
If your opponent isn’t using many buffs or hexes, you can probably swap this spell for something more useful fairly shortly. If your opponent isn’t using ANY buffs or hexes it’s not even worth casting this just to swap. If your opponent is using a lot of battlefield control spells you want to hang on to this spell until the enemy wizards are dead.
Magic missiles become less useful once most of your forces are in close combat. It’s a viable strategy to swap out Soul Quench once you think you don’t need magic missiles anymore. If Soul Quench doesn’t seem strong enough for you right off the bat (and you still want to blast things from range, you can swap it for a better direct damage spell. Lots of armored opponents? Take Metal. Demons or Undead? Take Light. Big Regenerating creatures? Also light. Lots of weak troops, Fire. Single low leadership monsters, Spirit Leech. If you are running a Bastiladon with the Solar Engine, having a potent magical missile on your Slan is less important since the Beam of Chotec is hard to beat.
I'd said Apothesis can be replaced with just about any Life spell. Then you'll get the ability to heal your units with the Life attribute AND whatever else the spell you roll does. If you REALLY need lots of healing, drop other spells for Life spells and hold on to Apothesis.
Hand of Glory is cheap to cast and covers the weaknesses of Saurus units very well. Since most LM armies are built on the strength of Sauri, you rarely want to swap out Hand of Glory.
The ability to get a free move for your units is so potent you rarely want to swap out Walk Between Worlds.
Tempest is a good spell to drop early unless your enemies are dominated by large quantities of low toughness troops or weak flyers. If there are NO low toughness units that make a good target, it’s probably not even worth casting just to swap it unless you end up with a lot of power dice your first turn and little else to do.
The value of taking magical items from kitted out enemy heroes is quite valuable. Not all armies rely on kitted out heroes so using this spell. I haven’t figured out enough to make generalizations about using this spell yet.
Fiery Convocation. If you get this spell off once your opponent will probably never let you use it again so you might as well swap it for something. My general play style gravitates towards cheap to cast spells so I’m unlikely to cast this spell even once. Because it’s so expensive you don’t want to cast this just to get rid of it. Burning thing should be your top priority, swapping the spell out should be your secondary priority (or tertiary priority if you relish the idea of forcing your opponent to waste his power dice shutting down your spell during his turn).
I put guidelines on swapping for spells in all the Lore Tacticas with

on them. Without breaking down all the lores here. First ask yourself what your army needs right now. Then figure out a lore with a signature spell that meets that need. Then after rolling a spell, figure out if the spell you rolled will help meet your needs better or worse than the signature spell and go from there.
It’s possible you’ll roll a spell you weren’t expecting that might make it worth changing your magical strategy on the spot. In my opinion those spells are:
Shadow: Okham’s Mindrazor
Death: Purple Sun
Life: Throne of Vines
Metal: Final Transformation
Light: Birona’s Timewarp
Fire: Flame Cage
Heavens: Comet of Cassandora