E isn't upgradeable with miles, but since your trip is within Region 1, W would be acceptable.
# For travel within Region 1 (North America, Hawaii, the Caribbean and Central America) upgrade awards can be used with booking classes C, D, Y, B, M, H, Q, V, W, S, K, L and T.
# For travel outside of Region 1, upgrade awards can be used with booking classes C, D, Y, B, M and H.
http://www.united.com/page/article/0,6722,1150,00.html