Well here is great work around for you !
Step 1 : goto above tab in SSRS configuration wizard
Step 2 : Press Delete and remove Key
Step 3 : Use below command in cmd to take a backup of the key
RSKEYMGMT -e -f c:/mybackups/myRSkeybackup -p StrongPassword
Step 4 : Restore the key that you just backed up and it will run with your new password.
Enjoy :) SQL !
