It also probably depends on the specific FAN plan details.
Through work, I have 2 options for a FAN discount. Don't ask

Anyway, I was on one that had about a 14-15% discount. The other one was offering 20% off. So I was finally able to make the change.
Ended up costing me money. The lower discount applied to the whole plan. The 20% was just to the base plan.
This wasn't a Mobile Share plan, but it may act the same way.
In my case, it was under $2/more a month, so it wasn't worth the bother for me to try to change it back.
Also, in my case, I couldn't find information posted about the details until I changed, and was able to look at my bill.