The answer to that is slightly complicated.
The spells in the core book, by themselves, are static, but if you roll well when casting, you can use Boon Points to increase various attributes.
In Libram Novus #1, I introduced a spell creation system. Not only for creating new spells, but it is also designed in such a way as to provide a method for altering already codified spells on the fly (sort of like in the manner you are asking about), not to mention actually creating/casting spells on the fly.
This is (unimaginatively) called the Spell Base system. If you look at the spell descriptions in Novus, you will notice that each spell has a Base listed. The spells in the core rules were all created using the Spell Base system from Libram Novus #1.
Personally, I think that this makes the Novus spell system better than the HARP system because of the simple fact that the spell creation rules for HARP were not finalized for HARP until way after the core book had been published, so the core book spells didn't really mesh well with the creation rules.
