It may be possible to take a completed set of -8 wings and use them for a -7 - but not the other way around.
If you look at the -7 plans (see DWG 10A [detail "C"] and DWG 38), the difference is in the rear spar. The rear spar on the -7 gets trimmed to a smaller profile. So the material on the rear spar exists on a -8 wing build - which allows it to be removed and profiled for a -7 application. This is why I believe the -8 wings could go on the -7, but not vice-versa.