Okay, I guess I didn't do a very good job asking my question. Under "Prolonging Spells" (YS259), it says you can extend the duration of an already active evocation effect. What I'm asking is, can you do something similar with thaumaturgy? As an example, say I have a ward on my apartment. When it's duration is about to run out, do I have to cast the whole thing from scratch again? Or can I do something like what it has for evocation, just using power for duration, and that's it?