KEDR change derivation rules

Hi

I need to change existing derivation rules of one of the characteristic in COPA.

1: However when i am in change mode in KEDR i am unable to change source (greyed out )

So do i have to delete the old rule step completely and create a new rule?

Will there be any issues if system is live and i am trying to change derivation rules?

Appreciate your help

Regards,

Reddy