JHS will do a rough conversion for you. It will NOT do any PL/SQL code for you. You will also be stuck with your existing data access design, for better or for worse. If it were me, I'd use the converter as an idea of how to build the screen in JHS, and then build it up from a sound business components model. You will be needing to tweak the JHS settings to adjust what is generated. You will also need to add your custom PL/SQL logic.
Don't treat this as a silver bullet to magically convert your forms application to ADF..
Agree with Brad. Note that Oracle Forms support will NOT end in 2017.
Check out this post with some more considerations around Forms2ADF: