There is no reason to believe that any particular location that can do the upgrades will or won't be better than others.
That's because you will encounter different CMs at any location on different days and at different times of day.
Some CMs have great training and experience. Others do not.
The method of getting the correct price for the upgrade that you want is knowing ahead of time exactly how much the
transaction should cost.
Then, if you are told a different figure, nicely ask to speak to a supervisor or just nicely decline the offer and go elsewhere to do the transaction.
To find out the exact proper cost for your particular upgrade, just ask on this message board.