Hi Amnesiac, welcome to NST
The drive letter choice is "greyed" at add/remove if EasyBCD detects where the boot files should be, to prevent users from pointing to the OS instead of the boot files. (the boot files point to the OS).
(earlier releases didn't do this, and most "problems" had this cause, so Easy was changed to prevent it)
If you have a good reason to change it, you would use "change settings".
Glad you're OK now though.