Not speaking about the plus but does the iPhone 6 case fit for the iPhone 7?
Unless it is a loose fitting case, my assumption would be no. The detentions are slightly different, not including the lens dimensions, button locations, and absence of the headphone jack.
iPhone 6: H 158.1 x W 67.0 mm x D 6.9 mm
Iphone 7: H 138.3 x W 67.1 x D 7.1 mm
Goodluck!
7/7 Plus will fit 6/6 Plus but not the other way around (because of cameras).