How does a change in geography (national/local) affect TV performance?

If a spot airs nationally (national or dual feed etc), the lift is calculated from the same national or dual feed geographies.

So when a shift is made from 50% spend in national/dual feed to 90%+ national/dual feed, what is the impact?

The impact is unique to the geographies.  If a shift is made, it is a discontinuous shift...or an apples to pumpkins shift.  The buy with 50% local is unique because the traffic coming from the three buckets of organic, paid brand search, and SEO are unique for the several minutes on that Wednesday night...and comparing it to a national buy is simply different as those visits for a certain period of time coming from the entire country (or half the country with East and West dual feeds) are unique to that geography and time. 

If a mistake is made in the post log file sent to C3 for which geography the spot aired in--that is a crucial mistake because it affects how the baseline is calculated, so the geography in the post log file needs to be accurate.

For clarification of national or dual feed coding, please visit:   http://help.c3metrics.com/article/109-adding-tv-data