I just had the problem that i wanted to change the project-type of an Eclipse project to ‘Java Project’ — it was a normal project before. This are the steps i executed:

1. Close the project
2. Go to the project directory  (e.g. using Terminal application)
3. Open .project file
4. Go to the XML node <natures> and insert <nature>org.eclipse.jdt.core.javanature</nature> as a child.
5. Save the file
6. Re-Open the project

That’s it.