As you can see on screenshot i have two curves and empty has a path constrain. The constrain below should take over another constrain at frame 200 but nothing works and thats not only one problem. Once i am adding second constrain. empty jumps back and behaves very weirdly srarting to make crazy movements and nothing helps - applying scales or changing axis. Thats a bug?
I think you just need to have your empty at the centre before you add the follow path constraints (Shift-S cursor to centre, then select empty and shift-S selection to cursor). It seems to work OK for me.