It's possible to do it. I just did it today by calling 888-876-8381 and the Rep. changed me from Sero Unlimited ($60) to Sero Premium 500 ($50).
Edit: This is what I did: Went to Sprint.com/employeereferral , got the My Way plan for $70, then called 888-876-8381 and tried to see if a Rep would do a TOL, they said it was not possible and she noticed that I was in a pricier plan (My Way $70) and switched me to the Sero Unlimited for $60. Yesterday, I called the same number again and asked the Rep if they can move me to the Sprint Sero Premium 500 for $40 + $10 Premium data, the Rep asked me why I wanted to move to that plan and I told told her that it covers all of my needs plus it's cheaper.