Send email on user role change

I need to use BR to send an email to a user when that user's role is changed. Is this possible, and if so how would I detect if the user's role had changed? In my case, the role would include 'subscriber' (ie when the role changes from just 'authenticated' to 'subscriber').

I know how to send the email, it's just the Condition (or Rule) that I can't get right.

Any help appreciated.