@edvin I think you have 4 strategies
1. Separate repo (tornadofx-8, tornadofx-9)
2. Separate Maven project (tornadofx/tornadofx8, tornadofx/tornadofx9)
3. Profile (id=jdk8, id=jdk9)
4. Branch
For option #4, I've seen some that create a 1.0 and 2.0 right away. The master is almost irrelevant. You can also keep 1.0 as the master and periodically sync with a 2.0 branch. In that latter case, one day the 2.0 branch would take over.