When you reach a level where you can choose to upgrade your base Magical Class into an Advanced Magical Class, you will find certain Advanced spells require starter spells from either the Common or Arcana sphere. It is wise to remember that once you advance, you will no longer have access to new spells from your base class sphere. Instead, you will only be able to strengthen the existing base class spells you have by purchasing them at the magic shop. All new spells you are able to add will be designated by your Advanced Class and the new Sphere you have evolved to.
However, when someone tried today to increase the number of slots of a spell someone already knew, but of a sphere they no longer had access to, Des blocked the transaction on the grounds that the sphere was not available.
Now, I know the site is somewhat out of date (a topic that's worth a discussion of its own, but let's not derail here). There are, however, three possibilities here:
1) The site is correct, and there's simply a mistake in Desdaemona's coding.
2) The site is correct, but the limitations of the sale protection script mean that the spell slots have to be added manually.
If that's the case, we'll just have to live with it, I guess.
3) The site is out of date, and reinforcing spells after advancing is no longer allowed.
If this is the case, then I think the site description should be fixed as soon as possible, because it easily induce players in error. It should be made clear that you cannot purchase any spell from the older sphere once you evolve, so players can plan appropriately. Otherwise they will evolve to an advanced class, trusting the site, thinking they can improve the slots later, only to realise once they try that they've made a serious mistake that can only be fixed by resetting - thus losing any bonus stats and increasing their reset count (which could end up in a level penalty in the future) through no mistake of their own.