From a farrier's perspective - what would you expect/want from a client who was opting to stop using you as their farrier as far as notice, explanation, opportunity to address concerns/keep their business?
Be honest if you are changing farriers. The current farrier will know quickly who you are going to be using and probably why so it is best coming from you in the first place. (It is a small world)
At the end of the day it is your horse, your money and ultimately your decision. The farrier should respect that.