I work for Sprint Prepaid Group. The person at the Boost Mobile store is absolutely correct. The rep is wrong.
If you stay with the current Prevail at the $35 monthly price plan or switch to another Prevail, then your plan will stay the same - $35 (the grandfathered plan). If you change the Prevail to ANY other android phone (such as the Samsung Galaxy SII), then the plan will go to $40 Android at the NEXT billing cycle date. At the time of handset swap, no additional payment needs to be made. $40 is the lowest plan all New Androids can go with shrinkage, and it has been this way since last year when the ZTE Warp came out.