You cannot remove a member from a group and keep the data on it. When you remove a member from a group it will have to move that data off to another member in that same pool, then remove if from the group databased and reset that member back to factory defaults
re: 5000. It cannot be upgraded past 6.x EQL firmware. That is correct. The firmware will not install on that controller.
When the PS5000 was still supported you could have it in the same group as the 6500.
Your option is to back up the data on the PS5000. Delete all the volumes. Delete it from the group. Then you can create a new group from that member if you wish and restore the data.