Amending up happens ALL the time. Its not just a threat. The reason most of the cases end up resolved is because the defendant generally realizes it will cost them A LOT more if they lose and either pay the fine before court starts or plead guilty. Once they do the math, defendants generally want to resolve it quickly before the prosecutor gets the chance to amend up! After all, the moment the matter is addressed in court, the applicable fine rate is now the 'statutory' one; not the 'set fine' rate. So, that in itself leads to a higher fine.
Usually the defendant has already gotten quite a good reduction (e.g. 49 over instead of stunt driving) so they know they are risking quite a bit if they proceed to trial. They are simply being greedy if they think they are going to get another deduction from the prosecutor. So, think about it---why would a prosecutor give them a discount when they have already spent court time, spent the officer's time, etc. Quite the opposite, most prosecutors love the opportunity to 'amend up'; its an easy way to get more out of defendants; especially on easy cases to prove like speeding charges.
So, don't fool yourself in to thinking that prosecutors are just bluffing----its as simple as notifying the defendant they'll be amending up and then requesting the amendment in court. There's no extra paper work for them. It doesn't require more than 20 seconds of extra work for the prosecutors and the money collected can sometimes be double!