I'm throwing my two cents in.
There is another alternative I've been considering. Cumulative spell knowledge.
The idea is that lower spells are used to understand higher level spells of a similar type. So, a fireball spell is an upgraded version of a firebolt for example.
Learning a fireball spell would be easier(less xp) if the wizard has learned firebolt. Once learned, the spell is never forgetten, just a subcategory of fireball.
The problem is that there maybe many spells that can't do this.
"Trading in" Wizard spells
Re: "Trading in" Wizard spells
Nevertheless, the underlying logic is sound. I quite like this idea. We could compose 'spell lists' - like those found in Rolemaster - of somewhat related spells. You can still buy any spell you want at the regular XP cost; but if you have the earlier spells in a particular spell list, you can buy the next spell in that list at a reduced cost, because you already understand the underlying concepts of that type of spell.
As you say, not all the spells could be related in that way, but I don't think they all have to be. It's an in-world explanation that makes sense of a potential game mechanic.
As you say, not all the spells could be related in that way, but I don't think they all have to be. It's an in-world explanation that makes sense of a potential game mechanic.
Re: "Trading in" Wizard spells
It all depends on the situation, but I think it's something that can be handled with Director discretion, and doesn't need mechanics really.What if a wizard decides that glowing eyes is a really cool spell when they start their adventure, but as they grow, they decide that it doesn't have as much uses as they thought it did?
For instance, since the pandemic began, I've been running an AFF campaign online, but none of the players have played AFF before.
The party's wizard initially picked Firebolt, but after only getting a hit against orcs every third or fourth shot (accounting for fizzles and dodges), or being forced to spend 2-3 extra mana to guarantee hits - she quickly became frustrated with Firebolt and asked if she could switch up her spells by dropping Firebolt and another level 1 spell and taking Forcebolt instead. I just let her do it, and she's been having more fun.
I kind of agree with her though - Firebolt is a terrible spell! Since targets get a dodge, Firebolt will miss monsters with skill 7+ 50% or more of the time (unless compensated for by spending more mana). Since it also doesn't ignore armour, you end up having to spend 3-4 times as much mana casting Firebolt for the same combat effect you would get from using Forcebolt (no dodge, no armour).
The only real use I can see for Firebolt is as a "budget multi-spell" - for wizards that want to be able to "light fires" and "blast things", but don't want to spend the points to take Burn or Ignite AND Forcebolt as well.