Hi Anthony,

I would rewrite the letter like this:

To Whom It May Concern,

My name is Anthony Arnold. I am a client of your agency. I understand that
you recently added another client to your caseload. This is very nice.
However, I would like to know why my time and others on this same caseload
have been changed without previous agreement or acknowledgment. I am a human
being who lives a life and have things that I accomplish on a daily basis. By
changing my time without prior agreement, you are in effect telling me that
my time and life is not a valuable part of your schedule. I do not like this
new scheduled time since it doesn't allow me the ability to do what I would
like on my own time frame.

I suggest that in the future, you call each person on the caseload and ask
for their permission to alter or permanently change their time for xyz
reason. If the time change is not acceptable for various reasons, then work
around that accordingly. You can't change everyone's time arbitrarily. I
believe that goes against the ADA laws and the rights of the individual in
general.

I know that your time is valuable, but so is mine. I, too, am a human being.
Please respect my rights and feelings.

Thank you in advance for your time and help in this matter,

Anthony