I have ran into this a number of times and finally gotten around to posting issue.
I have notice the cash register entry needs to modification the security name on the linked Cash Register, but Quicken give pop up error that the change needs to be done in the linked Investment Register. Which if you make the change there it does not carry offer to the cash register?
I image one has to delete the two transactions - one in each register then readd, which is a pain.