New role assigned on user account update
Just found a way to do it without coding. Works well so far.

EVENT: After updating an existing user account

CONDITIONS:

1) Data >> List contains item - LIST: "account-unchanged:roles", ITEM: "Role A" (Check NEGATE)

AND

2) Data >> List contains item - LIST: "account:roles", ITEM: "Role A"

ACTION: Send mail

Ref: http://goo.gl/Pcz2wN