That's a tricky question, with a difficult answer. I have not tested this:
First: Navigate your way to the "mods" folder.
Once there, right-click the ars magica mod (mine is called 1.7.10_AM2-1.4.0.009.jar) and select open with winrar.
Now winrar has opened you'll have "am2", "assets" and "META-inf" folders there. Open the am2 folder.
Scroll down till you find the "spell" folder and open it.
Now scroll down till you see "SpellUnlockManager.class" and open it
Now, as said earlier, this is just speculation, but it seems like you'll be able to modify the unlock requirements for the silver line spells there.
Sorry for late answer, hope this helps.