What are Grandparents called in upper Maine?

Hmm. Upper Maine might bring you into the French zone and the French descent folks might be more like Quebec and use grandpapa or grandpère as a familiar term, perhaps even papa or papi.  My dad was from Aroostook County (potato country, northeast of Millinocket so southern Aroostook) and we called his parents granddad and grandmom, for what it is worth.  I grew up in southern Maine, and even though it was a french mill town, it was still different from northern Maine.

EDIT. well, in truth, we called my dad's father "grampy-doc" for obvious reasons and to distinguish him from my mom's dad (granddad); her mom was Nana.  Dad's mom was grandmom.

look up the various French terms -- in the older days, families and/or marriages used to move across the border in that area [in either direction] but near everyone spoke both English and French [although one would be less developed than the other, depending on family usage].  -- grand-pere

Grandfather and Grandmother is customary.