Glad you're on board with updating to 1.8.
Here's what *I* would do, because I tend to prefer commandline tools over GUI ones.
1. I would start with piping all the manifest files through sed...
for m in $(find . -name MANIFEST.MF); do sed -i -e "s#J2SE-1\..#JavaSE-1.8#" -e "s#JavaSE-1\..#JavaSE-1.8#" ${m}; done
2. Run a build to see what breaks with the updated manifests from 1.4, 1.5, 1.6 -> 1.8.
(If you don't like sed, there's prolly a way to do the whole thing in Eclipse, but that'll likely have to be done one plugin at a time.)
3. Then fix all the compilation complaints in Eclipse.
4. You'll probably also want to bump all your plugins/features from x.y.z to x.(y+1).z to avoid breaking old consumers.
The easiest way for that is to run this:
mvn -Dtycho.mode=maven org.eclipse.tycho:tycho-versions-plugin:1.0.0:set-version -DnewVersion=5.6.7-SNAPSHOT
But bear in mind that it expects ALL plugins/features to be the same version, and if your project doesn't roll that way, it won't bump everything in the reactor in one fell swoop, so you'll have to run it in each and every plugin folder. Or just edit your MANIFEST.MF and pom.xml files by hand.
Nick