According to JBDS 8.0.0 Beta2 release notes:
The default Forge runtime starts on pressing Ctrl+4. To change the default runtime, click Window→Preferences, expand Forge→Installed Forge Runtimes and select the check box of the runtime that you want to be default.
This is not true. I changed the default to 1.x, started Forge using Ctrl+4, and it still starts 2.6.0.