From my experience,they will CHARGE you $15.00 as a THANK YOU for upgrading.Any time you make the slightest change in your service,whether an upgrade or downgrade,they will attempt to stick you with this.However,if you don't mind a little haggling,they may waive the fee or credit your acct. for the amount,as they did w/me.
Beware that they will try to charge you an additional $15.00 on top of that as well{an activation fee},just refuse it & do it yourself.
Also,if you don't mind changing your phone #,they may give you the 1st months service FREE,as if you are a 'new' customer.I know 1st hand they used to do this w/non-smartphone plans,however,the ANDROID or BLACKBERRY plan may be a different story.