Crane Flow of Fluids Technical Paper #410 -
see note for example 4-14:
in the absence of test data for increasing elbows the resistance is conservatively estimated to be equal to the summation of the resistance due to a straight size elbow and a suden enlargement.
i.e. Latexman posting is confirmed by the most well known handbook on the subject.
to determine which coefficient to apply you need to know if the flow will be IN thru the 6" (sudden expansion) or OUT thru the 6" (sudden reduction)
you may find some formulae in internet - or acquire the Crane Technical Paper #410.
saludos.
a.