Because Oracle does not let one alter an Index in this way, one needs to take the following actions.
WORKAROUND 1:
If one wants to do something like that, one needs to send the script to the editor (with a drop command), edit the script, and run it.
WORKAROUND 2:
If one would do it all via GUI, one could right-click and choose “create like” on the index, make whatever changes desired and drop the original index.
STATUS:
Fixed in Toad for Oracle 12.8
