Clicking cast spell should open up a spell menu with all prepared spells sorted by level, with a simple way to upcast them using a higher level slot. Without making each and every upcastable option it's own icon. Simple +1, +2 etc tabs or something which would change the entire layout into +1 level versions of all prepared spells. Same interface could be used for metamagic.
NWN2 did it well, I would check that out.
