Hi Dave,
Just a note to say that 'attach to a running target' will be supported for Segger debug connections in the next release of MCUXpresso IDE, expected later this year.
In the current version 10.0.0 'attach' is only available for LinkServer and P&E debug connections, so changing the OpenSDA debug firmware would be another option.
Within the IDE; Help -> Additional resources -> OpenSDA Firmware updates, will take you to a site where you can select alternative OpenSDA firmware. I believe the default for the FRDM-K64 board is CMSIS-DAP (LinkServer) firmware so perhaps you have already changed to J-Link.
Yours,
LPCXpresso Support