Assigning Roles Signup and Email Verification

Hai everyone
I am new to UM. I have a strange problem / request.
I want to do the following :
1. Allow users to signup on to my site. On initial Signup I assign them a simple “view only” spectator role.
2. I have opted for the email verification, so that the user gets an email to verify and activate his account
3. Now after step 2 above, I want to change the User Role. That is, when the User email is verified and account activated, I want his role to change to Participant (or any other ) where the user can write posts.
Is this possible ?
Thanks
Suresh