my tactic is a bit different: i try to swap spells with the lores which have a hex or a buff as a signature, because i feel like i only need these kinds of spells lategame, so i priorize life and shadow, and eventually beasts and heavens. If i get the wrong spell i take the signature and i stop swapping with that lore, otherwise i keep swapping with the same lore until i have to pick the signature and then i stop. Obviously i can swap with other lores if needed, for example metal if i have 1+ enemies on the battlefield or death for a dangerous enemy character, but i usually don't. Regarding the swappable HM spells:
-drain magic: usually swappable, unless your opponent has one or more RiP spells with high casting value
-soul quench: swappable, even if you probably need to cast it 3-4 times if your opponent has too much chaff
-apotheosis: swappable expecially if you swap for life, consider keeping it if you have lots of monsters
-HoG: never swap this, really. If you swap spells with shadow combo HoG and miasma in the same fight
-WbW: it depends on deployment and your army list. Swappable IMO, but make sure to use it at its maximum potential before it goes
-tempest: definetly swappable. Probably the first spell you should cast
-arcane unforging: this spell is a drug, once you've casted it you can't stop, but it's swappable even if sometimes it's worth casting it 2-3 times to cripple dangerous opponents or remove nasty magic items (BotWD, hellheart, empire speculum and so on)
-fiery convocation: swappable. Just cast it one time, if you can, and let it go. You could use the power scroll to cast it easily and if your opponent isn't able to stop you he'll waste his next magic phase to beat the original casting value. Cast it on a horde or a deathstar (i prefer the second option since there are good chances of miscasting and i really don't want to blow up my slann for some skaven slaves or goblins)