eclipse-java-roll-back-jre
Download the older JRE and install it (on Windows it's often put in c:\Program Files\Java\jrenumber )
Right click on a Project -> Properties -> Java Build Path ... Remove the "new" jre
Right click on a Project -> Properties -> Java Build Path -> Add Library -> JRE System Library -> Alternate JRE
(If you've changed the default JRE for Eclipse you can choose Workspace default JRE (jre6))
Right click Project -> Properties -> Java Build Path -> Add Library -> JRE System Library -> Alternate JRE -> Installed JREs
Here you can add the new JRE you installed in step one (basically just need the location where it's installed)