Hi David,
Simple answer: MCUXpresso IDE is the IDE for LPC and Kinetis going forward.
Kinetis Design Studio will not be updated going forward, and KDS v3.2.0 is the final release.
There is no need to migrate existing KDS projects, as you can continue to use it. For new or future projects it is recommended to use the MCUXpresso IDE.
I hope this helps,
Erich