It is a complete waste of time and resources to have dogs searching for marijuana, especially in states where pot is legal recreationally and medicinally. Even in states where marijuana is not legal recreationally, those resources can be better used finding dangerous explosives and hard drugs, as well as locating missing people and finding fugitives. If you are worried about drug trafficking from country to country or state to state, have a few K-9 units placed at the borders for that purpose alone. However, otherwise randomly assigning K-9s to search for pot is a complete and total waste.
Lets face it, at least here in america we know that drugs are a problem. Drugs lead to guns, crime, gang warfare, and even deaths from over dose. What would the valid point be? In case anybody had medical marijuana? Well if you do need medical marijuna than all you have to say is "Officer, I need to tell you something, I have medical marijuna". What the officer should say is this" do you have a perscribtion for that?' And if you really did need it then you should have it perscribed by your docotor. What is my oppenents point I'm unsure, but to do debate this i hope he has a damn good one.
I think the re-training of the drugs dogs should only come AFTER marijuana is legalized. The law is the law. They'd be breaking the law if they trained drug dogs not to smell it before marijuana is legalized. Plus, I'm not 100% sure if it is even possible to basically "untrain" a dog.